The math gets funny as everything is interrelated.
If you take the powder charge of a 180gr round and put a 155gr bullet over it, you get more velocity. It's usually not a linear relationship, so the recoil is usally less than the 180, but not much as the 155 is now moving faster.
A good recipie in general for a soft shooting round is to put a heavy bullet over the minimum charge to get the velocity you want, and to have that powder be towards the faster end of the burn rate charts. Somehwere fast enough that all/most of the powder is burned by the time the bullet exits the muzzle. The mass and action of the firearm interact with this in such a way that the last fine tuning is a holistic process. That involves playing with spring rates on most guns, and various other things that are platform dependant (e.g. on 1911s, you can play with firing pin stop profile, recoil and mainspring wieghts, shock-bufs, main spring housing shape, etc).
As a general rule with recoil springs lighter weight means that more energy stays in the slide, and you get more flip but a gentler return to battery. A heavier spring rate means more energy is absorbed byt he spring/frame, and gives more of a pushy recoil.
In a heavy gun, heavier springs make the mass of the gun work for you more in the disipation of recoil by taking more advantage of the inertia of the heavier gun.
heavier mainsprings will generally result in torque similar to flippy/snappy recoil as the energy transfer changes it's axis of motion in order to cock the hammer of the gun (non-issue on striker fired guns).
For the M&P, I'd probably jsut stick to a faster burn rate powder, and try the 14lb recoil spring when it comes out Given that it is a light gun with a striker.