Plastic rack pistons are good.
They act as a mechanical fuse in case something goes horribly wrong: instead of fucking up the gears, which are tedious to replace as they will need a new shim job (yes it gets easy after some practice but still a nightmare if you've never done it), they'll break instead. Easy peasy fix as you just put a new one in and you're good to go.
Now, if you want to go for an extreme 50+ rps dsg you kinda have to go for a metal rack otherwise you'll be changing pistons every 3 magazines.
I don't see any problems in running a plastic cylinder head either, as long as it seals properly. One thing you can do is to radius the gearbox, it's a quick and easy job everyone can do and it involves filing down the angles on the gearbox shell where the cylinder sits (towards the barrel), plenty of guides online on doing it. Cylinder is the same. Make sure it has the correct air volume for the barrel and bb weight you'll be using.
I also wouldn't bother with a tight bore unless you're looking at increasing the energy output, they're marketed as "precision" barrels but they don't affect accuracy that much. A good hop setup does. Invest money on a decent rotary chamber (doesn't have to be expensive, and plastic chambers more often than not are better than metal ones. Rotary because it can be fine tuned much more easily than the old style chambers with gears in them), hop rubber (Prometheus or Maple Leaf) and tensioner.
Mosfets.
It depends on what you want. I personally would go for a gate warfet, but it is not internal to the gearbox and if you're tight on space it might not be a good solution.
I would avoid the Titan, not because they're bad but because there is no point in putting a Ferrari electronics into a wonky ass Punto. It has 300 different functions and 299 of those you'll never use.
Maybe the Aster if gate fixed it or one of the options I mentioned earlier.
Either way you'll want a battery protection mode (shuts down the replica when the voltage goes below a fixed threshold that you can set depending on what battery you run), the basic feature of using the trigger contacts as a basic signal input (high voltage batteries will wear down standard contacts much faster, the fet bypasses them pumping current directly into the motor, the contacts will just tell the fet to send/not send current), and the electronic fuse (in case the gearbox locks up it shuts everything down instead of fucking up the box itself or the battery).
And seriously tell that guy to fuck off. £200 to fix an airsoft gun is robbery. Unless you asked him to slap a titan in there.
