IMO it's not gonna do all that much, if anything if your motor is basically stock. I have a 90 SC with a 70mm BBK/SCP TB, it might have added 5 HP but I really can't tell any difference. Maybe if you had the heads worked, replaced the MAF with a larger one, and changed the fuel injectors to larger ones it would be worth it. Also if you have not changed the MAF it's only a 55mm on an 89, so installing a larger TB would really do nothing for you, since the MAF needs to be at least the same size or bigger than the TB to work properly...I think that's how it goes, but I could be wrong. Either way a good cold air induction system, or a raised top (if you don't already have these) would get you more bang for your buck. I'm no mechanic, I'm only speaking from what I've done to my 90 5 speed, and from what I've read, and been told. They are needed when you perform heavy mods to the motor to get the fuel/air mix into the cylanders faster than the stock system can provide. Not to say you can't do it, just that you might expect more than what it will get you. Good luck with whatever you decide.
-JS