Matt, You are at a slightly different elevation and running a tuned pipe. Your needle settings will be different than running the stock muffler. If for some reason your tuned pipe setup isn't timed right it will be stuffing exhaust back in and will run lean and over heat. Could be header length, convergent cone location, stinger length etc. Different engine exhaust port timing requires different pipe timing as does going from glow to gas. Heat and flame spread also effect tuned systems.

Too bad you don't run it with the factory tested exhaust to simply see how it acts, could be it doesn't like your setup. I had the OS 160FX on a Jett incowl turbo muffler. Gained 1000 rpm on a Bolly 18X10 carbon but never landed with the engine running. Tried richening the high, the richening the low, leaning the low and all possible combinations. Ran great on the ground but would either blow the plug (lean) or just quit in the air. The engine had baffles all around it and 1 to 4 air in to out. Ran perfect on both a Jtec Dual snuffler and a Bisson ST4500 pitts.

Of course if it runs fine now that it's back, Hobby Services fixed something but the world may never know. If it tunes like the 160fx it will never run correctly if the low needle is lean.