There are a couple of things you can do to "tune-up" an efi, even if there are no codes set. With the throttle closed, measure the voltage on the TPS. To really run it's best, a GM EFI will need that TPS to register .5 volts. If it isn't, adjust it until it is. If your TPS isn't adjustable, modify it until it is. This is really critical - you wouldn't think so, but it is. Also check WOT setting to make sure it is around 4.5-5.0 volts. If it is outside that range when adjusted, replace it.
Next, your manual should have a procedure to reset the IAC valve. If the IAC valve is a little off, it can cause problems with driveability. Check your MAP valve with key on/not running, and with engine idling. Make sure it is within manual specs. A good MAP valve will even compensate for a vacuum leak, so make sure it's good.
These are just the main things to check for. Hopefully that will cure it.
I bought a scan tool last year, and it has helped me to straighten out some cars that wouldn't set any codes, but that didn't run very well. Knowing things that make a EFI car run poorly and being able to check them on the fly made all the difference in the world. If you can get your hands on a scantool (a real one, not those jumper wire code readers) Those Turbo Buick guys have a website that helped me with all the little items that ended up making big differences, you might want to check it out.
http://www.gnttype.org/techarea/techpage.html
