I've had good luck with the Mac's pipe to date. I do believe it might actually be making a little extra power, but I haven't proven that to myself yet. I also have a second engine (that's still breaking in) using a Bison Pitts type muffler. Equally good luck here, though I've heard of some others having to block one of the 2 outlets to get enough muffler pressure. That's not been an issue for me.

Evo w/Bison Pitts was in a Phoenix Decathlon, and although it flies fine (with plenty of authority), it's not my favorite plane. That hardware is currently being transferred to an Phoenix Extra 300 that Santa dropped off recently. By time I get this plane sorted out, the engine shold have enough time to do a fair Mac's vs. Bison Pitts comparison. First engine is in a .46 U-Can -Do.

In my experience, you'll see a noticeable difference in the amount of black crap as the engine breaks in and you get the carb adjustments ever closer to the perfect settings that can only happen over time. -Al

BTW, the fuel running out the carb issue, noted a while back, turned out to be a really tiny crease in one of the internal diaphragms. It wasn't allowing one of the "flappers" to close properly. That part replaced, we're back in business.