OK, so I decided to give ABS off a go with Rocket at Tuskuba, before I start though, allow me to bore you with some completely tedious background information from my own perspective about the pad and ABS;
I've never felt at any disadvantage with a pad while playing Gran Turismo. I remember one occasion in GT5 where I was in a series with the FGT alongside players i was familiar with in other series, and I noticed the smooth throttle application they were capable of with a wheel gave them a sniff advantage over me - not a lot, maybe a tenth or 2 a lap; that was the only time i noticed any disadvantage. Indeed I think pad has always been the advantageous of the two in FWD applications where quick direction change and point-and-squirting is key.
Thing is though, ABS off is incredibly difficult to manage with a pad. With throttle application I find you can manage traction nicely by a series of quick 'blips' on the throttle trigger, brake application doesn't seem to work like that on GT games though and a smooth application is required - this is something that's basically impossible on the controller triggers and only marginally better on the analogue sticks.
This all being said I gave the Rocket 10 laps blast around Tuskuba with ABS off expecting the worst, and honestly, was incredibly surprised. The only time I experience a lock-up was into turn one, and because of the extra work I was able to put into the front tires under braking they heated up far better than when ABS was on, this in turn reduced the understeer and by lap 5 I managed a 59.2! Finishing my 10 laps I can conclude that on a fast lap I was faster than when I had ABS on, but similarly I was no where near as consistent in OVERALL lap times with it off - I also think tire wear would increase significantly for me over a race length though I can't really confirm this over 10 laps.
In conclusion it's certainly more viable for me than I initially thought it would be, perhaps my lack of consistency can be boiled down to lack of experience with ABS off, or perhaps it's just a far harder thing to do with a game-pad, most likely it's a combination of both. If I have the time this week I'll do all my practice with ABS off, and if I think i can be consistent enough to not ruin anyone else's race by Sunday then it will remain switched off.