I was pretty fast with a DS3 before I got the DFGT, nowhere near as fast as now but I wasn't un-competitive and was always able to drive reasonably and be spectacular at overtaking.
https://www.gtplanet.net/forum/showthread.php?t=131934
There's results from a TT there, those drivers were insanely quick, DOUQA is the fastest guy I've ever raced with, even faster then those PURE guys. Dessy 182 has qualified for the Euro GT Academy finals and the National finals 3 times. Those guys were fast, they helped me get faster by quite a bit and I once almost got pole against Jamie-C and PJ-GM amongst others, some of which were in that, at Monaco, but I binned it.
This was on a separate site, which is now deleted from the Web so it can't be found, but I finished 2nd IIRC. Then I got my wheel in Christmas 2010 with GT5, so I never had GT5 with a DS3 so it was probably easier to get used to, but still tough enough, couldn't handle the Civic '97(starter car) in the Sunday Cup.
I had to change to Professional on the wheel and I went for some Arcade against the crappiest AI setting until I got used to the settings(had looked up before hand and knew that Pro setting and FFB at 7 was supposedly ideal) I gradually improved, doing the odd races from site to site, not really much, until the summer where I was in a lot of series and really improved myself. Dug up this post which showed my improvement just from racing in different series.
https://www.gtplanet.net/forum/showthread.php?p=7027418#post7027418 Same specs, cars etc. And by now I'm probably another half second better.
And then once you get the time you learn ways of going faster and saving tyres.(you will destroy tyres for a bit once you start
)