When James' Rockout demo was released to the public there where what? 10 reported incidents where out-of-date drivers was the main point of failure for low performance (the other was the rotating background killing the fillrate)?
Keep in mind though, that pretty much everyone who tried those demos who you heard from were blitz programmers themselves.
Programmers tend to have higher end machines than your average mom&dad that go out and buy a cheap dell with onboard intel video, and who still consider their computer up-to-date four years later because the guy at Best Buy who sold it to them said it was the best thing since sliced bread. However, those people DO happen to be the big target market for little shareware games, which is what many people use Blitz for.