Outdoors in the cold...I'd stick with sub 350fps (better yet, 300fps) with 0.20's..
Outdoors in the regular season...I'd just buy a new G&P complete mechbox. I've seen issues with every mechbox....custom built cheap, customs built expensive, factory cheap, factory expensive, etc....
Bang for buck...it's cheapest if you figure out how to reshim and swap the spring on your existing mechbox. You're looking at about $30 worth of parts (if you need to buy a new spring and a new baggie of shims).
The out of pocket cost is one thing...and the JG/cheapo mechboxes are definitely attractive for that. But consider the "down-time" factor. The time it'll take to have a gun down and wait for a new mechbox, then the time/hassle to install it, fingers crossed that it works with out issues, etc... Not to mention how much it absolutely sucks-a&& to go to a game just to have your rifle barf on you.
In general...you do pay for quality. Either in the quality of the parts and/or the quality of the assembly/tweaking of it. Bang for buck I've found the G&P complete mechboxes to be a good deal. Makes me sick to spend $300+ on a systema...can't stomach the builds of the JG/cheapos. I have seen the various gear/piston/etc...issues, but I've seen the same on lots of others too...
Also keep in mind that your rifle is not just a single part. When you change the mechbox (either in FPS/Joule power or in ROF) you need to think of what else is required with your motor and battery as well. If your motor is going to be stressed pulling a much higher load at a different torque ratio...it's not going to last long. If your battery is mismatched for your setup, you're going to starving the other parts of power. If you goose a motor with a big battery and push it beyond it's optimal capacity...you'll have a dead motor sooner than you'd think.
Fortunately there's some pretty simple guidelines to those parts as well...a good motor (G&P M120, Guarder Infinite Torque Up, TM EG1000/700) and a good 8.4v/9.6v battery (Intellect, GP, Sanyo) are easy to find.
I cannot tell you how many guns I've seen that "don't work" but it's really just a matter of an insufficient battery or cheapo motor that's been killed by being pushed too hard.
Best of luck,