An M150 is a great way to kill a Version 2 Mechbox. You'd not lose much velocity with an M130, but it would be much easier on the mechbox. I'd also swap the silent head set for a Bore Up cylinder set. You may even want to consider an aftermarket reinforced mechbox like Systema or Guarder.
If you're looking for accuracy above all, MOSFETS and Systema motors aren't going to do a thing for you. You want a tightbore barrel, a better hopup unit, a better hopup rubber. With the huge springs you're looking at, I'd also consider Torque-Up gears and an aftermarket piston, too.
In short, with the spring suggestions you made, if you're not willing to drop a pile of money on reinforcing other parts, your gun is going to wear itself out in short order. Remember, the Japanesese did not design these things to be upgraded, and you're asking to push the boundaries of engineering here.
Probably the easiest upgrade, if you're not skilled at repairing mechboxes, is the Systema Magnum motor & complete mechbox kit:
http://www.redwolfairsoft.com/redwol...l?prodID=21323
Add a Prometheus Perfect Metal Chamber hopup unit:
http://www.redwolfairsoft.com/redwol...l?prodID=21268
Add a Laylax Strike Chamber:
http://www.redwolfairsoft.com/redwol...l?prodID=16884
Get a good hopup rubber, like the Firefly Kurage "spicy", or the KM/Head RH65. Finish with the tightbore barrel of your choice.
However, afyer buying the gun and upgrades, you;re getting close to Systema PTW territory, price-wise. Maybe it's a better option?