lets say a freind of mine has a 70 mm mass air would it harm performance if i throw it on a 91 gt that is completely stock? except a cold air that may also go on
Agreed. It's kinda hard to speculate on something like a CIA, they have been known to give totally different results on two identical applications. Some cars love them, some don't, and some are oblivious. Either way, while it can't ever hurt to add a little more cold air to the mix, it probably won't give you anything you'll feel without some window stickers.
Bolt ons usually are most impressive when you do a few at once, i.e. CIA, pullies, RR's, gears; then you'll feel something- and most of it will be the gears. I'd say get it, and save it till you have some other stuff too. It will be most satisfying that way. Don't pass up a deal