ok, what i have fund in some testing, and from what racers have told me is that the stiffer the spring the more torque.
the softer the spring the more rpm
same with the brushes, the more surface that is in contact with the comm, the more torque
the less surface that is in contact with the comm, the more rpm
from what i have heard about the putnams, is that they are good, but i have never tried them. I usually run orion, trinity, or reedy in my monster.
i have found that when you are running the orion's you would need to run a little softer springs to get the performance out of them.
with the reedys and the trinitys, i use the same springs for each brush, on fast open tracks i run maybe red/black. on medium tracks, i might run red/green, on tight and twisty tracks where punch is despratley needed, i like to run purple/red
but it is all about gearing, more than the brushes, to make sure that the motor does not get over heated, and keeps it's cool