I still don't really understand why everyone is so preoccupied by how fast something will go. Although it is a valid question, it just wouldn't be high on my list of priorities.

Generally speaking, the faster something goes, the less runtime you get and the more quickly things will get hot. I started off with 7XL in my conversions. I'm now running 8XLs. Negligible loss of speed, noticeable reduction of heat. You said, 12 to 14 cells. On 12 cells, I'd say that the 8XL will be on the mild side, although you could gear it up. On 14 cells the 8XL will be pretty powerful.

Just keep in mind that I don't mess with 1/10 scale trucks. All of my stuff is 1/8 scale and is heavy, so you just have to keep my comments in perspective. Personally, I'd try to find an 8XL for the Revo. If I had to choose between the 7XL and 9XL, I'd get the 9XL. The speed of the 7XL is always fun at first, but the 9XL will reward you in the long run with more trouble free running.