Only trouble i've had with my fg 2speed is the catch pin in the 2nd gear carrier snaps with lots of power. Easy upgrade, i just punched it out- drilled out the hole and pressed smooth part of a shock screw thru and cut it off with a dremel, it's about 1mm bigger and i've had no problems since..in 6mths.
It shifts the same way the nitros do with a centrifigul dog that gets thrown outward and catches the pin...pretty basic but solid design. you do need steel pinions though and i run plastic spurs no probs.
I like having the option of gearing without having to compromise either acceleration or top end.
I reckon using skinnier steel gears a+b with the hub machined into the gear so it's one piece and catch pin pressed into 2nd gear/hub I reckon there's room (I've seen some Fg steel one piece hubs/gears on ebay.co.uk last week. wish i had access to a machine shop