For on-road or oval racing I'd run the lightest springs you can for max RPMs. It helps on some motors to drill the brushes as well. There are some scientific benefits to the magnetic fields etc from doing this but basically it reduces the brush contact surface area on the comm which means they'll stay seated with less spring pressure.
There are many racers who cut or dremel the brushes to change the timing, but IMO this is only changing the dwell (in automotive terms) which is the amount of time the power flows through the brushes (points). It also reduces the contact patch, allowing lighter springs, as will grooving the brushes. Regardless of the explanation, through experimentation I have been able to get 3000+ extra RPMs from Co27 motors by doing this. Mod motors seem to require more spring pressure than stocks. Experiment with different brushes as well as each motor responds differently.
As in all racing, it's wise to buddy up with the local fast guys and get them to share a tip or two each race day.