Hi Philip,
I am the only GB user of one of these currently, so you can see that most people opt for the CVT!  It has some advantages and obviously some disadvantages which I'll try to capture:
Advantages - Engine/gearbox unit is more compact than engine/CVT - Also the 2:1 reduction means you can use a much smaller sprocket on the rear axle which again helps with a compact install - Trouble free, none of the issues that you can have with CVT belts etc
Disadvantages - As you say, you can't get into the power band as readily and this is most apparent on a standing start and exiting very low speed corners

I try to mitigate the low speed power differential by using a selection of drive sprockets and I swap them according to the circuit and conditions.

I would suggest if you want reliability and a neat compact install go for the reduction box, if you want performance, go for the CVT.

No I don't, I'm afraid

That is a subject in its own right and I dont think you can get anything to help other than a higher reving engine with more torque at the 4000 rpm area.
You can cut the weight's in half (looking from the casing) and fit a stronger spring in the clutch and that raises the rpm before the clutch bites.
That what I have gone for and I have tuned the engine so that it will rev to around 9000 rpm. Unfortunatley, that give more than 10hp so I have to moderate it. I can fir a limiter I supose so I done get more than 10hp?

You need to knock-up a dyno to check it out. I am working on one now and when its done (?) I'll take some photos and explain its workings.
It will be portable and any cyclekart will fit so I could bring it to a meeting?