This will probably get locked but I have a 3kgt that did 305mph
and My 'yota GT1 maxed a 373mph(still trying to replicate)
Many people have broken 300 mph at the test course, with rules restricting them:
https://www.gtplanet.net/forum/showthread.php?t=69076
Some others have gone twice that speed using glitches, but I'll let you dig up that thread yourself if it interests you.
I reached 307mph in my Nissan r92cp. I could probably reach a greater top speed if I spend more time adjusting the gear ratios.