Coupla little problems with single-plane manifolds. 1) The idle is going to be inconsistant - it doesn't settle immediately in at 850 RPM's. It kinda wanders done for a few seconds, then settles in nicely. 2) They are all cold-blooded by design because they don't get heated by anything nearby. You need to brake left-footed in order to keep the engine lit until it warms up a bit.
If you can love with these little peccadillos, they can add 40-75 HP depending on the rest of the rngine. I got about 75 peak HP at 5,000 RPM's over the Performer RPM. Plus I love the look.
Brian