From what I have read in official writtings(Magazines, interviews) the race spec engine was capable of 800 bhp with the least restrictive restrictor plate. The versions that actually raced though were limited to between 660-680 bhp. As far as I have been able to find the only difference according to TVR between the street version engine and the race version engine is the street car doesn't have the restrictor plate, and has some tweaked engine mapping to optimize the added air flow.