Excellent work mate and a very comprehensive well written guide 
My old “trick” for those types of springs to gain slightly more pressure at full extension was to give them a gentle stretch to around 15-20mm of the original length before refitting.
Not ideal, but works surprisingly well when there were no other options available. 
Would you be able to upgrade that motor, get a bit more torque out of it, but as you pointed out that bevel-not-bevel gear is definitely the weakest link.
Wonder if those old BF gears for the Uzi/mp7 would work.
Oh yeah, you can get high torque, brushless, all sorts of upgrade 370 motors. The benefit of which is the grunt to pull a bigger spring.
But…
In my experience they aren’t worth the bother… you don’t notice much improvement… and you’re bang on when you say the gearing probably wouldn’t take it anyway. 
Awesome write up and investigation work mate. LOVE IT!
And really glad to see it came up to a decent fps.
I had feared it would be loaded with proprietary parts limiting the possibilities and this is good confirmation of that.
Shorten inner barrel, spacer the spring, clean and grease. About all that would be worth doing to it by the looks of that crown gear too.
I am surprised to see only three gears, though it might have been four like the LDT MP7, but I guess it is what fits and works in the space provided by the receiver so cant complain really.
Again, awesome work and review. Thank you @Friendly_Fire
Doing the gel gods work once again.
Well, Beemer’s was getting 170FPS stock, the one on X-Force’s review was similar. Mine was only doing about 140FPS… maybe it was a Friday blaster.
But maybe if you’re starting from that higher FPS of 170, possibly just shortening the barrel might be all that’s needed to get it up around 200… and no need to split the gearbox.
Then again, add a spring space and it might get up in the 220 range.
Hopefully other members will do their own choice of mods and come up with an even better result. But I’m happy. 
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Makes 230 fps, out of the m92 a pretty good achievement…
Mind you, it has super solid metal gears…
Still…small barrel and volume…
Good result for an EBB pistol.
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Withe the Uzi…could you shorten the barrel, from the other end, and keep the hop up.?
Or just be done with it, and install a after,market one…?
I get 210 out of my MP9, similar size gearbox.
But the design of that one allows for a really short barrel, so better VE. I know when I was modding that thing I trialled longer inners and the chrono readings went backwards. 
That short barrel in your M92 would be part of the reason it’s doing so well.
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If I got my hands on one it wouldn’t be difficult to model up one of those to be printed.
That barrel does look like it’s been machined down, like from 9.5 to 8.5 outer, not dissimilar to some GBB pistol barrels.
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It’s turned down at the hop up end to give a 10mm OD with the rubber sleeve on. Easy to replicate with a bit of lathe work and a file.
I still reckon an adjustable slim hop up will be a better option.
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Quick update…
Just found out the inner barrel can be shortened at the t piece end. The collar is removable after all.
Understand though that popping off the collar and shortening the IB at the t piece means filing a flat on one side of it to allow for a detent strip in the collar, there to revent rotation and keep the hop up aligned.
The receiver also needs modification if you shorten this way. The factory diameter of the barrel hole at the t piece is 9.0mm, it will need to be opened up to 9.5mm to allow for the lack of machined groove or the shell halves won’t close up properly.
The posts above have been ammended to reflect this.
Still think it’s easier to shorten at the business end.