Welcome to the boards!
I too, am running an '88 Renix 4.0 and I hate to tell you this, you've pretty much done all the cheap performance upgrades already. Besides a bored TB of course, which is somewhat hard to find for the renix, but you can obtain one on ebay.
Then there is port polishing, cams, etc. but that is a lot more than a cheap bolt-on.
The best thing to do however is either go the supercharger route (if kits are still in production) or the stroker route which many have taken successfully. Neither will be cheap, though.