I'd say go with the 9XL. It will treat the Mamba Max a bit better and you can make up for speed in gearing. I've tried the 7XL, 8XL, and 9XL in my trucks and like the 9XL the most for general purpose bashing and racing. It won't be the fastest setup, but it will be lightning quick with lots of torque. It'll make that Picco look like a toy in the low end department and won't be too shabby on the high end either.