I am very pleased with my MAB but, I share your feeling that "it isn't fast enough"! I can tell you with all seriousness that I can go much faster with a group of strong bicycle racers on my pedal bike.
Low grade bicycles are not designed to go 40 mph. High mileage at 25-29 mph puts a serious strain on the rear wheel. I'd put a smaller sprocket on the rear but the stress on the rear wheel would be too much, I fear.
As a very experienced bicycle racer, I've seen high speed wobble take riders out at high (40+mph) speed - and this is on high $$$ bikes that are designed to go fast! Get a cheapo bicycle up to 40mph and anything bad can happen!
I'd love to go 40 mph on my motorbike but that's asking too much. Keep in mind what these are: highly efficient, short range vehicles, with a very limited speed.
Good luck to you.
Timbone