E - Nissan Silvia S14 (!)
D - Pontiac Solstice GXP AND Audi RS4 (for WTF Road and Mission Street etc)
C - Pontiac Solstice GXP / Audi RS4 for the Gridlock Drag
B - Bmw M3 Sport Evo (e30)
A - Bmw M3 GTS / Murcielago Roadster for the Downturn Drag (and Drag in general)
S - nothing, sorry there NEVER were and are real S cars for IGC. real S performance would be Cinque, F1, Agera/CCXR, Veyron, R8 LMS, Murcielago SV, Carrera GT or Aventador -and that's it. That is the Club 400...
You can of course buy the Murcielago Roadster and tune it to 751 or sth, you can tune the Mustang 2012 Boss up to 782 or similar or the LFA around 800 (LFA currently NOT available for IGC) BUT all these cars are 20-30kmh slower than the real S Class cars.
Nissan S14= 200k
Pontiac GXP= 500k / Audi RS4 600k
M3 e30 Sport Evo= 850k
M3 GTS is around 1.9million /Murci Roadster ~1.8million
I rec the S14 because it has good handling, corners well and has better Nos than the VW Scirocco. Plus, you only need the Street tuned blue Turbo and bam, you're on 249 OA. The S14 technically is the same as the S15 but one Turbo perfectly fits, so easy to go.
All the other cars do have strong Nos too, that is important if you wanna win. 90% of all players on restricted tracks drive such cars.