by 7 do you mean zx7?Is that just on their bikes? I've also heard similar (3%) for the cars, but I've had the GPS in both my M3 and my 328is and both have been spot on up to their respective limiters. Both had roughly stock diameter tires on too.
I've found the speedo on my 7 to be far more accurate than bikes that have the speedo ran off the tranny. Once I get my fuel pump issue fixed, I'll try to get out and see what kind of error I've got at higher speeds.