Well, there are simply too many variables to attempt to answer that question...
A simple test though, find a motor, like a Reedy KR where the 766 is the brush to use.....which is unserrated. Serrate it and put it in the motor.....I can pretty much guarantee the motor will be slower than the non-serrated setup. The 766 just does not like to be serrated, thats why its not sold that way.
It really depends on the motor/spring/brush combo to determined if a motor likes serrations or not. Sometimes they love them, sometimes they don't. Its a lot of trial and error.....
And sometimes just a simple hole or slot in the brush will make a faster motor than a serration will....but likewise, sometimes none of the above will make a faster motor.
At Paved nationals, Peter Robinson's stock motor that I tuned used the same brushes in round 3 and 4, along with all 3 a mains. The thing just kept getting faster and faster each run with those old brushes....and after the 1st a main, they were gone....all I did was put a light skim cut on the comm and clean the motor and the brushes....
While I never like to send people away from this forum, I do reccomend you check out
www.rccars.com and look at big jims motor forum.....tons of info. Jim can be unique at times, but its easily worth the information.
Later EddieO