The fastest record for double bass is Tim Waterson, I'm not sure of the exact number but it was very high, around 255-265bpm. The bpm thing people are talking about, what it is that alot of people who are into metal drumming say for example 215bpm. They mean 16th notes at 215 bpm. The fastest double bass in a musical context that I am aware of is for George Kolias of Nile that has a song with 16th notes at 264bpm. Derek Roddy of Hate Eternal has some songs that are 16th notes at 250+bpm.