I have a GXH50 copy, the HS142F, on a SBP shifter mount with 4G, so I've played around a lot with the rpms. My 4G's clutch is engage-happy and hits at 1900-2000 rpms, causing me to roll-and-pull to start. What I've noticed is that ~3750 the power really does pick up. This matches the Honda graph, where the GXH50 makes ~1KW of power and over 1.8 lb/ft of torque. On my setup, I'm turning 6K rpm @ 30, so I'm usually under that.
The Max-Torque clutch in the Q-matic offers a different experience through the rev range than a shifter setup - it's less direct. That has some good points and bad points depending how you look at it, but the Q-matic is definitely a nice setup, IMO. It's very soft on delivering power to the rear wheel. I put over 200 miles on my brother's GXH50 + Q-matic setup last year and was always suprised by it's 20+ prowess.
As for the Honda governor, many people prefer to have total control over their throttle themselves. Personally, I think the governor is dangerous hooked up considering our application. Besides, the engine looks so much cleaner without it!