I just ported SC on an intercooled 1998 GTP yesterday. His rotors had about 15% of the coating left.
He his mods consist of:
CHEAP DRYER DUCT HOSE (you know that aluminum foil duct that is very not smooth)
Harland Sharp 1.9 rockers
3" back to the stock y where every thing is stock after the y also.
Two major downfalls for him are his pointless intake setup, and his stock exhaust manifolds. Surprisingy he has no KR.
He was seeing a max load of 9-10 psi before the ported SC (ported/polish inlet, and lightly ported outlet).
With the addition of the ported SC, and lightly ported/polished 98 TB we saw his boost jump only 1/2 lb, but we didn't scan for KR because we were fighting a vacuum leak at the T we installed just before the MAP senor for his boost guage. Removal of the T fixed the vacuum leak and hesitation but we couldn't see boost on anything but his stock DIC boost guage (it maxs out bone stock). His Power Tuner wouldn't show boost levels on the laptop either.
The little time that we did have the boost guage hooked up we were peaking at around 10.5 lbs, but I suspect boost was being dumped due to the vacuum leak and bad readings at the MAP sensor which tripped the codes P0107 and P1107.
So yeah, it'* definitely safe to say that with all that coating gone you are losing a 1-2 lbs of boost, ***very possibly more***....
Reason I think you are losing more is 96BonnevilleL67 has very good coating on his SC rotors ( I think his SC was replaced as it isn't the correct color of SC that 1996 M90'* were) and with only a 3.5" pulley, Ported SC (inlet ported/polished, outlet lightly ported), lightly ported/polished TB, his 3" fenderwell intake (wrapped now), 180* t-stat, and wrapped his crossover pipe he is seeing around 11 psi, but then again he doesn't have 3" exhaust.
My old 1995 SLE SC I found to be missing about 50-60% of the coating from the rotors, which could very well be why that car wasn't as fast as it should have been at the track (between that and the SC gasket blocking the SC outlet).