I used to race Quads Honda 250 R and you get one done up right it will take anything out there. Banshees, 450's what ever. I have to give the honda 450 alot of credit. Motor wise, My friend bought a 450 dirt bike about 2 yrs ago and tore it apart to make a 450 quad. Cuz the 450 wern't out yet. Talk about a wicked fast quad head is flowed and piped thats it. Nothing can tough it on the track. But there is also about $15,000 into it. total on the whole bike. custom frame you name it.
So it is hard to say what the fastest is.
Just my 2 cents to.
By the way My 86 250 R is for sale if anyone interested. lets say only thing stock is frame and wiring.