Think of a level 10 mag. You increase the spring tension to make it easier on paint when operating at 300fps and sacrifice a bit of efficiency. If you reduce the spring tension, you will make it harder on paint at 300fps, but should gain some efficiency.
The regulator can go down to quite a low pressure. The normal operating chamber pressure of a level 7 mag is about 350psi. The regulator can go quite a bit lower than that such that you can get 170 fps shots. I don't know what the chamber pressure would be at that range, but I suspect it would be quite low. So, the regulator shouldn't be a problem in its current configuration.
If you do use a lighter bolt spring, you can reduce the operating pressure of the mag. A few things will happen. The ball will get hit harder when a shot occurs. The power impulse will be more abrupt. More low pressure air will dump from the chamber during each shot. That is a good thing for efficiency, especially since the power impulse would keep the bolt fully forward for more efficient acceleration of the ball. The bolt hitting the ball harder is a bad thing, because it does that at a higher speed. This can also damage the next ball in the stack. If you lighten the spring too much, then the reset becomes an issue as well. As it is now, the biggest resistence in the reset is the residual air in the chamber. If you operate at a lower pressure, then the friction forces and the mass of the bolt become a larger percentage of the return force needed. They are not as consistent and can change depending on dirt or maintenance activities which could affect the springs effect on the bolt mass.
It can be investigated. I've often wondered about this too, just for curiosity.
Except for the Automag in front, its usually the man behind the equipment that counts.