It may be the slimline muffler. I had almost the identical problem. I had my 160 on Hanger 9 EDGE with a bisson pitts muffler and the engine ran great. I then Purchased a slimline pitts with smoke chamber and the engine ran horrible had a lot of deadsticks and poor transition, especially in the midrange.

I then put the bisson back on and the engine ran great. The engine would only run at about 8100 with a Mejzlik 18X8 with the slimline but turns 9200 with the same prop and the Bisson muffler.I am using 15% nitro and 20% sinthetic oil. Altitude her is 6200 feet. Also using Perry pump.

I am not saying this is for sure your problem but it sure was my problem.