My expertise is in much larger engines, but I have to ask about the noise...is it mechanical, or intake/exhaust roar? On engines with very small cylinders, the valve lash can make an enormous difference in performance. Check the lash, check the spec and if you find that the lash is on the loose side or looser than spec, try tightening it to the tight side of spec. If the main jet in the carb is replaceable, go up one size and retest. if it's not replaceable, consider the cost of a new carb and decide whether or not to drill out to the next size (you sound capable of using good science). Unless the engine is truly not engineered to produce 6, then only a few things can restrict the power that badly. Do the spark plugs have any color at all? If not, certainly you can benefit powerwise by adding some fuel. I know how manufacturers are fast and loose with power numbers, but 50% off sounds like some sort of restriction.
Here's a good one-I had a motorcycle once I thought too slow, and checked everything. A friend of mine, called in frustration found that the throttle wasn't being pulled open all the way. Check to see that you're not working with 2/3 open carburetor.