I have run them on a couple different Chevy TBI rigs. My '96 Chevy with a 305 and my jeep 304. Both times I saw 1-2 MPG gains and what could be purely imagined increase in low end grunt (seat of my pants Dyno).
They were designed for carberators and thus will have a slighty positive effect for TBI as well due to the similar design. On a newer engine with MPI, they are a useless paper weight.
I probably would not have bought one for my jeep except I needed an extra inch more clearance under the TB. The impact on performance is so small that if you do decide to buy one, make it one of the last things you do. There are a ton of other things to spend money on first, or wait until a buddy gives you one.
With a great Jeep comes an empty wallet.