I was thinking about this whilst practicing using the GT06 round HSR... and I realised that it tends to actually force you to use proper driving technique, whereas other cars let you get away with much more...
I would say that in the current form (post-August update) the Ford GT 06 you've already been using is probably your best bet. You can turn a very good lap in it, but you have to drive it very much in the "classic" (real life) method:
1) Slow down as much as necessary on the straight, leaving little or no braking to be done during corner entry.
2) Use gentle throttle control to guide the car to the apex.
3) Once you get to the apex squeeze throttle down, don't slam it down.
(Note, in current post-august form I'm fairly sure GT5P isn't mapping this correctly. What should happen in the GT if you hit the apex and just slam the pedal is a good likelihood you'll spin up the rears and then spin the car. All you get now is gobs of understeer. However, either way, if you squeeze the pedal down the results are an appropriately handled (fast) corner).
If you do this the GT 06 will still turn a blistering lap even though it does tend to understeer pretty badly if you mess up your entry even a tiny bit. The problem is that you pretty much can't race it online this way because most of the cars out there allow later braking and can be "thrown" into the corners more violently. If you over-drive the GT into the corners you'll usually just understeer and wreck it, and if you drive it properly some idiot driving a different car (usually the S2000 it seems) will just slam into you because they don't have to start braking until much deeper into the corner and they can't be bothered to drive properly.
This really gets my blood boiling.