At a Tamiya Championship Series race earlier this year, the difference between GT3 (silver can) and GT2 (stock motor) A Main average lap times was about 1.5 seconds per lap, 16.8 seconds to 15.1 seconds. This was on what I'd consider a fairly technical track. Both GT2 and GT3 are tub chassis classes. The drivers weren't the same, but this should give you some idea.