OK, a whole evening of racing and no calibration issues. I suspect it was some weird software issue and not a wheel hardware issue given that restoring the hard drive got the calibration back without so much as touching the wheel. Fanatec support got back to me very quickly and have just asked me to keep an eye on it and let them know if there are any further problems. Happy with that and their fast response.
Anyway, my first impressions after two long evenings of racing with this new wheel (good points and not so good points). But for TLDR readers, I'll just say it is pretty good! For other readers, here is the detail: Firstly, the feedback fidelity - perhaps unsurprisingly - is a whole new level for me (for belt drive) compared to what I had with my previous 911 RS2 wheel. There are now sufficient subtleties coming through the wheel that I can take low speed cornering much more consistently which has made my lap times much more consistent. I can now really feel exactly the right amount of throttle to use to exit out of these slow and medium speed corners whereas with the old wheel, there just wasn't enough detail to take these corners consistently well. And trail braking into corners is much easier now as the feedback is giving me much clearer indications of when to get off the brake.
Compared to my old 911 GT2, I've dialed the feedback down from 96% in Race 07 to around 66% with this PS4 wheelbase, with only very minor changes of a couple of percent elsewhere in that sim's feedback settings. I'm running the wheel itself with 1080 degrees rotation, damper off, spring off, drift off (I used to need drift set to 3 on my old 911 wheel to be able to counter steer perfectly) and FEI to 030. I am really glad other posters here convinced me to go with this wheel rather than the original Elite, since I am pretty certain I would have wanted a little bit of "drift" on that original wheelbase rather than being able to have it off altogether on this one. And now I can appreciate how "natural" it is to have it turned off altogether. And now that I can see what FEI does, I consider that a critical feature rather than merely a desirable one. For me, anything much above 040 clearly overdoes things but 020 is not quite enough. That said, since the differences even with these 10 point increments is pretty significant, I'd love to see single digit increments down the track if that were possible. It won't stop other people using their previous settings! Personally I would have loved to experiment with settings a tiny bit below and above my current setting of 030.
Some not so great points with my new PS4 wheelbase: the flex in the wheel is a bit more than I was hoping for. I realise some of that is the "budget" rim mount of the Xbox rim, but there is added flex because of the extensive plastic used in the wheelbase itself. That is not to say there is not a fair bit of steel where it counts, but I can see flex is being introduced both where the rim joins the input shaft and where the wheel sits on the cockpit mount (via a three pointer mounting). But as others have pointed out, the effect of this flex appears to be negligible when actually racing to the point where I don't notice it. But it is a bit of a concern in terms of long term durability. Ironically, my old 911 wheel had less flex because it was much smaller, the shaft was much shorter and the small wheel rim was far less of a "lever". But again, this is a price point thing.
The other slight negative is that possibly due to the ribbed belt drive (or maybe because of the motor, but I suspect it's mainly the belt) is that in certain situations you can feel some lack of smoothness which I call a "cogging" effect. By "cogging", I mean a very mild version of what always annoyed me with the Logitechs with their geared drives in certain circumstances (though people often used the word "crunchy" with those Logitechs and crunchy is way over-stating things on the PS4 wheelbase). But it's a feeling of the wheel rotating through a number of subtle "detents" whilst turning it rather than being totally smooth. The most obvious situation where I notice this is when I have to counter steer a bit more (or more aggressively) than normal. If I am driving and putting the car into a very slight slip under power as is my normal driving style, then it is excellent, but it tends to be most noticeable if I have made a mistake, for example and have to get my foot off the throttle and make a more significant correction to "catch a slide" so to speak. You can actually feel this "cogging" effect on its own in Race 07 for example, if you just take the car out, drive it like a granny at very low speed and just keep steering the car left and right - you won't notice much other useful feedback at all but you can feel at the little detents as you move the wheel left and right. Not that you would do this when actually racing of course, but it is just a way of isolating the sensation that I am talking about. Mind you, every other wheel I have owned has also done this - Logitech G25, G27, 911 RS2, etc.
As for the Xbox rim, I really love it! No real negatives there at all. It feels fantastic in the hands and although I might have suggested the rim could perhaps have been be a little bit thicker, that would probably add weight. And from what I have been told, the heavier the wheel rim, the less the feedback fidelity, all other things being equal. If anything, I suspect the thinner rim than I am used to reduces any tendency to grip the wheel too hard - I've noticed that even in the most intense situations, my grip on this new rim has remained light and without any tension. So I think it is great as it is - and the gear shift paddles do have a very nice action compared to what I have been used to before - no missed shifts at all now.
So in summary, I am pretty happy with the new wheel and was relieved that I could integrate it into my existing Race 07 setup with hardly any changes needed apart from dialing back the feedback strength a good 30% in the game itself. I was concerned it might take me literally weeks to "tune" the wheel to my taste, but it just took me a couple of sessions. It helped that any changes I made - both in Race 07 and on the wheel itself - made very obvious differences to the way the wheel felt so if a change made things worse, it was very easy to tell and I could quickly move onto to trying something else. All that said, I think I actually have a touch of buyer's remorse for not going all out and getting the Clubsport wheelbase instead (still with the Xbox rim though). I suspect that might have virtually eliminated that slightly annoying cogging feel when the in-game feedback is not there to "overpower" it - mainly due I suspect to the metal construction, metal pulleys and flat drive belts without any ribs on them (though this is merely my speculation). Then again, I have not tried the Clubsport wheel so can't make a valid "smoothness" comparison. But here in Australia it is $230 more than the PS4 wheelbase. And in the scheme of things, in hindsight I think I should have forked out the extra cash just to get that last degree of smoothness in all situations. Oh well, we live and learn. But next time I will almost certainly get Fanatec's top of the line belt drive wheel instead of their mid-range one.
Finally, I am very impressed with the modern Fanatec drivers (and not because of the extra features offered). I haven't had to muck with Fanatec drivers for years - and years ago they were quite tricky to install with separate firmware and particularly pedantic set procedures almost like aircraft checklists! These new drivers are fantastic - so user friendly!