A&K SR25 Upgrades

[Caelum]

I'm in the midst of reassembling an A&K SR25 (CA, G&P clone I believe), with an extended V2 gearbox (V2.5), and decided I might as well put some new parts in it to help with consistency/reliability. I have a few questions regarding upgrades (they are bolded).

I already have a PDI 6.01 509mm inner barrel, with plans to R hop and shim the hop up arm, and install barrel spacers, so for now accuracy related parts are taken care of.

The gearbox is mostly stock, with a new cylinder head, sorbo pads, and a TM "EG 700 super custom" motor.

Gears: I will be swapping out the stock A&K gears for either a set of CA torque up SR25 gears or a pair of SHS torque up gears.
Which is brand is more reliable?

Gearbox shell: I have the option to change the gear box to either a stock CA V2.5, or a X-Power V2.5 gearbox.
Is this really necessary, since I will be keeping the FPS under 450? How is X-power as a brand?

Piston: I will be replacing the stock piston with SHS 19 steel tooth polycarb pistons, unless there is a better and more readily available piston?

Loading nozzle: I have been having problems finding a metal SR25 loading nozzle. It is about a centimeter longer than a standard M4 nozzle.
Does anyone know where I could find a metal replacement with one or more o-rings?
Would a G36 or AUG nozzle work?

Spring: I have a standard AEG spring in the gearbox right now, but I want to swap that out to a correct length spring.
Where can I find a spring that will work in this mechbox?
Is the TM EG 700 motor I have strong enough to pull a spring rated for 450 fps?

mosfet: Is a mosfet with active braking necessary for this build? I will be rewiring, and removing the fuse.

mock silencer: I don't actually have the quick detach mock suppressor for this rifle, it was misplaced long ago, however I would like to order a new one.
Is it legal to import mock suppressors? Last I heard, it was pretty hit or miss..

There are quite a few questions in there, so if you can only answer a few, or have some info you think I missed, please speak up!

[Wrath144]

I wouldn't bother with the gearbox shell. The SHS parts are fine. The motor would be able to pull that spring, but slowly. The nice thing about the motor is that it will be quiet. As for nozzle, take a set of callipers to it and see if you can find something comparable at airsoftstore.ca
The mosfet isn't necessary, but will definitely be an improvement. Active braking isn't a requirement for the setup you plan on using.
I think you probably got the SR25 that I tried to buy :P
I was just going to upgrade it to sell, so if you want any help with the gun send me a PM.

[Ninja_En_Short]

G36 and AUG nozzles are too short, you can get a SCAR-H nozzle and use metal saw and a file to adapt the length fairly easily.