They're exactly the same fit. My wife is a hobbyist upholsterer and has recovered at least four different sets of Fiero seats from different years using the same pattern she made from a set of '84 seats. Of course as you mentioned, there are differences in trim, and the early seats had holes for the speakers in the headrests, but otherwise they're the same.
While the Covers are Off, you can cut the 3" Holes in the FOAM and install 3 1/2" speakers( they are just held in place by the tightness of the Foam). the Sound comes out just fine even THROUGH the material. Done it several times, in Fiero's, Camaro's, firebird's