I run a brushed motor with my Reedy 4200 35C lipos and it's awesome. The motor doesnt' seem to wear any faster. I easily get 2 races from them on a 10x2 and when I ran them with my 19 turn money motor in the SC10 I only used up 2400mah in a 10 min race
You can feel when they start to go and it's probably best you stop there and then to avoid them dropping anymore.