Soo I did a short Testing.
First I drove the Cars in Stock trim all at Nürb GP/D (1 out lap and 3 fast ones):
Mercedes 550pp 980kg 295kw (400hp) => 1:28.055
Alfa 555pp 1010kg 324kw => 1:27.533
VX Cally 561 pp 1060kg 351kw => 1:26.881
Opel Cally 552pp 1040kg 329kw => 1:27.371
I tuned the DF to 35/55 each => all where "equal".
After that I tuned all to 295kw/400ish BHP (hope thats correct; if not I´ll do it again for 305kw because there are two different HP-Calculators I searched for)
and did the same test again:
Mercedes time didn´t change maybe I´ll get it in the 27´s if I test more
VX Cally => 1:28.094 (minor cutting issue at T2/4)
Alfa => 1:28.261 (m c i at T2/4)
Opel Cally => 1:28.613 (minor mistakes by myself => 0.3-0.5 seconds off)
=> Brett you have your winner
pp values are different but the Lap-Times are almost equal.
(Stock downtuned => no Turbo // DF 35/55 // RS-Tires // Grip Real // No Aids ABS = 1)