well, not exactly. The numbers Mark gave are nearly 3 fpe apart, and that's about right. A stock 89 will do 19-20 fpe, occasionally 21. I'm not quite so sure about the 99, but 17-18 fpe seems to be the norm.
After a TurboTune type tune is a completely different story. The 99's might get up to 19 fpe, but some of the 89's (but by no means all) turn into 350 mag's with a red stain

(just what is Gene stuffing into them, anyway?)
I'm not sure what the differences are inernally, but I'm under the impression that the 99 might have a shorter piston stroke. (Again, not sure on that one).