Now THAT's a pit bike
My brother has a Pitster Pro and that thing has done fine with almost everyone that's ridden the bike. The guys have ranged from 5'8" to 6' and 150lbs-200lbs. It's a fast a$$ bike but so unstable around the track when you're in 3rd
$600 for a bike like that is kinda cheap and I'd worry about how reliable the parts are or if you could get customer support if something happens.
http://www.pitsterpro.com/