I've got a Downey header on my 85 EFI, i would say i got between 5-10 hp max, just enough to get it going a bit better.  I also saw about 1.5 MPG increase too.  My manifold was cracked which was my deciding factor.  Im running 2.5" through the cat into a flowmaster, and have 2.25" coming out of it and it runs really good with the added backpressure.

Justin