What tire preasures are you guy's running on the track? Second I take it that you guy's are turning off the traction control?
Perhaps I'm behind the times, but I'm not sure tyre pressure is an option.
Definitely different tyre types, but perhaps not to the point of adjusting tyre pressures.
As for traction control.
Some, perhaps described as purists, or more likely simply experienced GT drivers who can handle their cars without the need of TC, will suggest driving without any TC is the way to go.
And probably, if you can control your car, lap times would prove this.
So, depending on your skill level, TC is a hinderence or a help.
TC will slow your corner exit speed, the higher the level, the greater the retardance when accelerating.
So if you can control your car, the lower the TC the better.
If you having difficulty, keep the TC on, at a level which suits, until you feel more confident to lower the level, which in turn should help you shave a few tenths of your lap times.
I remember some time ago, there was seasonal time trial using an X2010, and I think it was around Monza. I, obviously, used TC, and I think I set it at 5, which is, I suppose, quite high. But up to that point, that's how I always ran my Red Bulls. And it seemed sensible at the time. Even then I was well aware of the effects of TC but felt that the X2010 was putting so much power down that TC was a definite requirement.
So, I put a time in. I'm not that good, but I accept that.
Anyway, I thought, "I wonder... should I just give the X2010 a run out with no TC and see what happens?" Don't ask me why, I just did.
I was quite surprised really, and I beat my previous time (set with TC=5) by over a second. I practiced some more and I knocked off another half second. So it really is possible to control the most powerful car in GT5 by being gentle, and a bit more precise with the throttle.
I use a G27 by the way. I don't think I'd ever manage the same with a DS3... just don't have the same level of control with a DS3. In my opinion, of course.