they both accomplish the same thing, applying more return force to the trigger, their method of action is extremely similar(one pushes on the trigger directly while the other pushes on a rod that pushes on the trigger) so I think its safe to assume that their effects would be the same. If it was me, Id rather have the added force go through the 3 way to make operation of the 3 way more reliable.
Now the availability of a spring with a higher tension(K value) that is the same size of the current 3 way spring might be a problem, thats why I mentioned spacers earlier....pre-compress the spring a little and you will have your higher return force.
just pulled apart an msv-2 and I see no reason whatsoever that it wouldnt work. there are 5 parts excluding the body...
Rear screw->spring->o-ring pin->o-ring->activation pin
the spring is about .2 in. and is a cone shaped, extremely light.
Actually, I just tightened down the screw farther than what it was originally, and it increased the return force to the point I dont think there is any chance of short stroking....go figure
edit: you can turn in the screw about 1 3/4 turns past flush with the body and it still works fine, anything past that the rear screw begins to interfere with the o-ring pin. Anyone ever do this in their mag? I only have an ep mag so I cant really try it in a working mag. I have 2 msv-2's here and you can definitely tell the difference between the one thats turned in and the one that isnt. It would be really interesting to see what a different spring would do.