my tach is definately atleast 500rpm off at redline. rev limiter is set low in my car at 6000rpm ecause it hits it so often in first or second on mistake when the tires break loose in first gear as soon as you get into it and as it starts getting into the 6,7,8lbs of boost area in second gear. but my ford tach will read as high as 6500,6600 rpm and my autometer digital tach with built in shift light reads right at 6000 when it hits the rev limiter. its not that far off if at all at lower rpms though

 

Yeah, sounds like my tachs. Which one do you trust? I don't put much faith in the factory stuff and mine reads off at all rpm's. Factory tach will be at 2400, Autometer at 2100. I haven't ever checked at redline.
And I wouldn't mind having your problem of traction, sounds like a fun car.