One thing you have to keep in mind if looking for a manual car - the Tremec TR-6060 in the SS shifts much smoother than the unit in the GXP, which is pretty clunky. I would get the SS just for this reason, the improvement in shifting action is very noticeable.
I think my GXP shifts fine, much better than the GTO and Camaro before then. One thing I've learned about the Gen4 Camaro is that the factory clutch for it didn't fully disengage, and that made shifting difficult, and GTO felt to have the exact same issue. Aftermarket RAM clutch was just as bad. But the clutch from S.P.E.C. was the good one, made the shifting easy. I've been quite happy with the GXP shifting, that only 25 years later they've finally managed to figure it out at the factory.
So, is there any chance that the GXP you've seen had an aftermarket clutch that still had this issue with incomplete disengagement?