Fastest BL motor on 6 or 7 cells?

I run brushless in my Traxxas Rustler. I have two. One with the ss5800 and one with the Feiago 8s/8012 combo. I am not really happy with the top speeds any more. I want a motor to get me into the 40's on 6 or 7 cells, without using tall gearing. I run M2k's so that adds to the mix. I only bash so I do more running in grass which also makes the temps higher.

The reason for staying with 6 and 7 cells is because I recently got 4 pair of GP3300 for my emaxx that I also run in my rustlers.

Thanks in advance for the help.