Ryan- The diffuser isn't in the circuit at WOT because the needle (which controls midrange) is all the way out of the needle jet thereby leaving only the main jet to do the fuel metering chores. Remember that the main jet screws into the needle jet not a jet holder. The needle jet has a larger ID than the main but is subsequently blocked to a smaller diameter by the needle. Removing the needle leaves only the smaller main in the way. That's why. As to the adjustable needles, I'm making up a couple of batches of them for common moped carbs and when they're done I'll post all the info here. There is a fair amount of calibration involved in making the jigs etc. to make such a simple modification. Many Kodiak Research techs use these on their personal engines. Rotax and Bing never did make it standard on their aircraft engines because of all the bozos who don't/won't learn about tuning and no doubt would go out and promptly seize their engines or something. The Kodiak is a division of Rotax based in the Bahamas that produces their aircraft engines.
As far as tuning mopeds vs aircraft... two strokes is two strokes, the torque curve requirements are the only significant difference as a propeller puts a constant load on the engine rather than a variable load like our mopeds. A two stroke is a load sensitive engine which is a whole other chapter. If I told you when I first started messing around with dinky engines you'd know why I was naturally bald and I might not be hip anymore.