Actually I find most metal gearbox airsoft guns, If properly upgraded to 350 FPS and with proper hop up maintenance to be the longest shooting and most durable working upgrades for most airsoft OEM gearboxes. The Re-inforced gearboxes do absorb vibrations and the increased weight helps with noise dampening, but the spot that breaks on most V2 boxes was not really re-inforced by this upgrade leaving them just as susceptible to nose cracking as the non re-inforced versions.
Systema has a nice airsoft unit engineered to shoot 565 FPS and it seems to work and last....However this does not mean that all you need to do is put systema parts in your standard AEG gearbox and you can achieve this as well. Most gearboxes on the market are engineered with 400 FPS being the Maximum and 350 FPS being the recommended. Most US based airsoft gun service centers will not upgrade above 400 foot per second without voiding any warranty programs.
I prefer to upgrade my airsoft guns to the 350 FPS point and raise the rate of fire to 20 to 25 bb's a second with steel high speed gears. The Modular airsoft gears are fantastic and last a very long time when installed this way. The spring I use is one that will land my guns in the 330FPS range and the addition of a metal spring guide with a thrust bearing rounds the upgraded gun out to 345-350 FPS when using .20 gram bb's. Plenty of firepower for any airsoft game!
One final note on the subject...The new KWA 8MM V2 gearbox units seem to have incorporated what might be a solid engineered solution to mechbox nose cracking issues by modifying the actual nose of the gear box and creating an interlocking set of staggered fingers that seem to absorb the shock and vibration created by the piston. Not sure what the actual limitations are on these but one can contact KWA USA and I am sure they would be happy to provide the new gearbox specifications. Better yet just buy one of the new KWA M4's as they will keep you in the game sharing welts...Long Time!