From what I have heard the trofeo tires on the z28 are significantly faster than the cup 2 on the R. A couple test have the Trofeo running 1-2 sec faster per on the same car in a couple different test.
Now the mpss on the GT350 will be slower that the cup 2. On the same tire that 3 second gap could become one second or less with the GT350TP vs the Z28.
2 caveats.
1.The gt350 time was a tech pack car, track pack cars are as fast as a z28, according to things I've seen hinted by ford insiders, no data though.
2. If gt350 on mpss specifically custom made to be better for all conditions vs performance, they literally sacrificed some, is that fast, mpssc2's on a nice hre or ccw or even tsw rotary forged would put a comfy plush $55k gt350 ahead of a z28 drivers being equal.
Now throw that full tilt boogie racing catalogue of uhmw bushings at it, ans maybe a tune and some 3.90's, and Jesus, it would embarrass a z28.
Who cares about stock vs modded.