A little more info dredged up:

"With unrestricted motors:
Previous to the 9500 RPM rev limit rule, NASCAR Nextel Cup engines were producing close to 900 flywheel horsepower. This is when they were spinning up to 10,100 RPMs. Of course, the best teams were making this horsepower, and the independent/small teams were a bit behind.
Here is an approximation of horsepower numbers produced over the years:
2004: 875 (or more) fwhp
2003: 840 fwhp / 750 rwhp
2002: 825 fwhp / 735 rwhp
1998: 750 rwhp / 680 rwhp

All of this data is based on post-race dyno testing posted on jayski.com. All of the teams tend to underestimate and keep their real horsepower numbers a secret (which is why most sites tend to say no more than 800 horsepower)."

I seem to remember reading some years back that they ran very high Comp ratios (15:1 on the restricted engines?) but didn't see this on web.

I don't really follow NASCAR but the engine development/technology interests me. On any NASCAR sites at present there seems to be lots of discussion around Toyota engines (which were let in last year) - seems they have a material BHP advantage.
Quite interesting to see that most of the forum comments I read seemed to be along the lines of "tough luck GM/Ford/Dodge, try harder/back to the drawing board" - I would have expected more parochialism and whinging.