Most of crazy demo "racers" use speedhack to get such unbelievable top speeds.

You want real speed and fun on Multiplayer ? Seriously, buy an S2.
When I was Demo Racer (long time ago), I was playing with Tweaker and shit, but it's totaly useless when you have GTRs and Formula.

I don't think it's possible unless you get a grip hack to stop your tires from bursting.

I've seen many "Crazy" tweakers start from 0-200km/h in few sec without burning off their tires, probably grip hack will help you a lot but I don't think It's allowed in here, right?

Gravitation? You can use that program to adjust the grav, then set your suspension stiffer and boom, more grip. I wouldnt try it for multiplayer cause when I go to replay and i change the grav it gives me a 00S error. But fun to play with AI cars