With RepG39 and firmware 4.0 I did some decent prints with 70 mm/s (Feed rate) and 100 mm/s travel rate ( MK6 (accelerated) and SK35).
With RepG40 and Sailfish I managed to print at 60 mm/s (feed rate) without having problems. I am sorry, the difference is not so big.
I tried increasing the motor current. At forum posting somebody claimed that increasing Vref from 0.6 to 0.8 will increase the motor current, however, I didn't notice a difference (maybe the machine has become a bit louder?)
I also think that replacing the original motors by the newer ones (from Moons) will allow faster printing speeds.