Why? XL is rather expensive, like double the cost, the Mamba Monster will handle that motor more than fine! About the only thing you get with the XL, since that motor has a 6S limitation, is the data logging.
8s is perfect for the 1518 (1800kv x 29.6v = 53,280rpm without a load) IMHO. The datalogging and 5amp BEC should do my buggy right as well :-)
If I could only draw what I see in my head, then afford to build it, and finaly get to play with it...