Springs
On,y MY opinion;
The more power the motor uses, the stiffer the brush springs have to be. If your motor is going soft at 2 or 3 minute marks, it either needs stiffer springs or needs to go down one tooth on the pinion.
Red on + for stock TC is a good set up to me.
Green on - is also good.
Green on + might be a little light and the brush will be blown away from the com. this causes more arcing which makes for poor commutation. which = less power.
It has been said here many times, stiffer spring more torque lighter spring more RPM. Each has its cost. I prefer the slightly stiffer spring so the comm and brushes stay cleaner with less arcing and pit marks for better performance.
Green on + AND - could be a good combo for 1/12 scale because the power (V*A=watts) is less. But in a touring car, I would go with red/green or Red/red . Some in this forum have said purple/red on a monster for more torque. I have not tried that because I run Pan cars and Off road.