Good to see that people are starting to do some testing.
As you test, its best to pick your favourite track, a track that you can drive very consistent. It should also be a track that has a good mix of everyting. Fast/slow corners, long/short straights, hairpins, chicanes. I usually use Suzuka as it has all of this, but there are other good tracks for testing too.
To do an apples-to-apples comparison I think its also good to test all the cars with default set-up. The only thing I usually change is the gearbox and putting the aero to max both front/back as its closer to what we race with than default set-up. Lap times will be slower than your race set-up, but its a better comparison between the different cars.
Post your lap times and any comments to it. We have plenty of time to get it right for the scheduled mid-season PP adjustments.