I've done some back to back dyno tests over the years and I'd sum it all up by saying it is inconclusive. Sometimes the 4500 manifolds are better, sometimes the 4150 with an adapter are better. The difference is usually in the noise so I'm not sure if the gains are real or if it is just the normal test to test variation you see on the dyno.

Just recently we saw some really good results with the Mopar 337 intake plus a Wilson 4150 to 4500 adapter. That setup just flat out worked for some unknown reason.

I'm not a big fan of the M1 4150 flange intakes since Shep put that darn Thermobog flange on there. The M1 4500 manifolds have a really nice looking plenum. I can't say they make anymore power, but the 4500 flanged M1 intakes sure look like they should make power.

If you want a better answer you need to track down Jeff at Modern or Dwayne at Porter Racing Heads. Those guys have a lot more intake experience than I do.