I've heard that with the speration the throttle body will have to work harder to deliver the right amount of full. But, as they advertise, they are supposed to mix the air fuel better. But I've never personally messed with one. But I have heard nothing good.
They are well proven to be worth a few horsepower. Car Craft dyno-tested one on a TBI Cheyenne pickup years ago. It has less to do with mixing the fuel and more about improving intake runner length. I'm not sure what BigT is talking about, I don't see how it could cause the TB to work any harder.
I personally have one on my '91 K1500 pickup, and my brother had one on his '91 9C1. It fits. You may have to rig up something with the bolts for the air box, though. Most of those come with bolts set up for the trucks, which are different.