F310B
Of course the JGTC cars are faster than the DTM cars! They are both cool but DTM cars edge it IMO, largely thanks to the Mercedes 190E. The coolest touring car in the game (followed closely by the Alfa).
If you like JGTC cars I recommend the McLaren F1. Okay it's kind of a lot faster (though it raced in the JGTC in real life), but it has similar handling and is more fun.
Yes it did, and does too, but not in the spec from GT4. It had to be rebuilt to comply to JGTC/SuperGT rules, so probably more weight and less power for the Mac in Japan!
EDIT: I´ve done some testing now, just to see the differences. They are not great, but the JGTC car I tested was a little faster around a track than the DTM car I used.
This is how I carried the testing out: Cars: Nissan Motul Pitwork Z ´04 vs Audi A4 DTM ´04.
I used R1 tyres, and no powerupgrade besides an oil change, wich gave the cars 485 and 482 bhp respectively. No driving aids were used, but all settings were set up to my liking, wich is done to suit my drivingstyle. They ended up pretty similar by the way except that the Z packs a lot more downforce. Other than that, I used the trannytrick, just because I felt like it.
The testbeds of choice were Midfield Raceway normal, and Citta di Aria normal, with 5 laps at Midfield and 2 at Aria. At Midfield, the difference was pretty large, as the Z won by almost 2 seconds, with a time down at 1 min and 7 sec, and the A4 had a 1 min 9 sec time.
At Aria, the difference was smaller, only 0.6 of a second, but I may have done some mistake with the winning Z, so the diff may be larger in someone elses hands. Z time: 1´32.1xx and A4 time: 1´32.7xx
Conclusion: JGTC 2, DTM 0
But I still prefer the DTM cars!! Better looking, sounds cooler, and more fun to drive!