As far as HP goes, I think someone up there ^ already answered that one for ya... A CAI is only going to help HP numbers if it is the bottle neck in your engien combo. If you're running a stock motor, stock heads, TB, and MAF, the NO CAI is going to give you a HP increase. The induction system that comes stock on the Mustang is MORE than adequate for the combo that it supports.
As far as fitment goes, that will totally depend on the combination that you're intend to use it on. Blower cars, or unique intake combination will require a CAI that fits TBs set in other than stock locations. If you require moving the MAF out to the fenderwell, then that can be an issue when selectiong a CAI as well.
For a bone stock 89 LX, it's a waste of money. It would be better spend on a 3G upgrade and later on and elec fan or something.
Heck... at that point, I might even invest in a larger MAF and call it a day. No HP increase from that either until down the road when your mods make the stock piece a bottle neck.
Buy once and plan for the future. If you want a CAI for looks, then great! There are some nice ones out there. If you want it to add some HP to your basically stock engine combo, you're spinnin' yer wheels bro.
what about the anderson power pipes...they must do something if people put them on their cars.
They DO, do something on engines that need to induct more air than the stock induction system is capable of supporting.
My KB setup for instance LOVES unrestricted inlet air. To the tune of a 4 inch tube even! Tossing a stock mass air meter inline it would kill at LEAST 30 HP.
CAIs (like exhaust), don't ad hp to an engine combo (the stock piece is already a CAI). Instead, they free up the HP that is already there. Larger, more efficient mills require more air than the stock diameter setup can handle efficiently.