The Dutch. The Dutch know what RPMs can be achieved because the Dutch tune the fuck out of their shit. They build drag puchs, sprinters, and they document the fuck out of it. If you want really really detailed technical info, register and read http://www.puchforum.net. The only catch is, it's all in Dutch.
They've also got stuff on youtube, but it's mostly just them riding or testing bikes on their "testbank." 20.9 hp is hard to argue with though.