regardless of displacement hp is limited
I get it. They were making about as much hp as the 600s?
Yes...in a nutshell, down on HP compared to weight. The AMA made concessions to Buell/Harley (as the only American bike manufacturer) to try to attract bigger crowds to the races. Earlier pure-bred race models like the XBRR were based off modified air cooled Sportster engines - so they were slotted into a class based on a performace index. Simply put, even with the added displacement they were uncompetitive.
The newer model(s) like the 1125RR use liquid cooled Rotax/Helicon engines - those bikes race in the 1,000 cc class, but they're somewhat overmatched.
Basically, Buell's got the formula down for the chassis, but the engines suck? Seems like Harley didn't want to kick out funds for R&D of topnotch engine, being that it seems that Buell was to be the sport bike arm and MV Agusta was to be the exotic arm of Harley...seems like Harley's upper management is only good with Chrome and Twins, and doesn't give a shit about Fast, Sport, and Fours. Like their business model only includes what they know and only what they know. Their attempts to diversify have seemed to fail or falter do to half assed doing stuff.