400 to 450fps? That's too hot for most fields, unless you keep it on semi...
Not quite sure what your love affair with Systema is, but you could do what you want with mainly Modify parts.
Use the parts listed in this set as your rule of thumb:
http://www.airsoftparts.ca/store2/in...oducts_id=1061
The only thing above that you need to get specifically for a SIG is the air nozzle. Everything else is generic version 3. Based on your barrel length (433mm tightbore), a Modify S110+ should be slightly over 400fps. The Modify S100+ in my AEG (509mm tightbore) is 400fps on the dot.
Since the above is a high torque set, get a high torque motor. A 9.6v mini will pull that, get a 1500mah mini if you're worried about battery life.
It might be worth your time to throw a Sorbothane cylinder cushion in there as well.
Edit: Beaten by LUTNIT. =P