SSC Aero is now the world's faster production car, according to the blurb it can do around 257MPH so it seems to go a little faster in PGR4.
Edit: Apparently...
Wind-tunnel testing indicates that the Ultimate Aero could possibly reach a top speed of 437 km/h (273 mph) given appropriate transmission gear ratios. The base Aero, however, should only reach around 386 km/h (240 mph).
http://en.wikipedia.org/wiki/SSC_Aero
thought that guy was using a glitch or something (like WhiteRoser's infamous "GetBurnoutRevengeTakedowns.exe""
