Modelwork Monster upgrade 8mm gearbox for V6 - here's the link:
I'm pretty sure the Monster gearbox was just a TM gearbox with the bushing holes milled to 8mm. http://www.ghost-airsoft.com/product.php?id=201112232037414213
. You can even see the "leftover" part of the oval bushing hole.
Given what they were doing with the TM gearbox 5 years ago, I think with the parts available now, there should be little to no issues using the TM gearbox, unless you're using an aluminum piston and piston head. I bought a used TM P90 where the person had used aluminum parts for both (and the air seal was terrible when I opened it up, so it made those impacts that much worse), and the gearbox had cracked on one side, and the other side was failing as well. It happens, and I ended up replacing the gearbox shell with another TM one. Moral of the story: don't just use aluminum pistons and piston heads without protection.
If you look through the uber high speed P90 party thread, you'll see that plenty of normal TM P90 shells have plenty of abuse put on them: http://www.airsoftretreat.com/forums/index.php?topic=75327.0
. And, I'm sure if you look around, you should be able to see what kind of abuse some of the E90/ACM P90 shells have put on them (though TMs seem to be the base of a lot more projects that I've seen).