How much battery do I need?

Speaking in terms of systems, higher voltage systems will draw less current than a lower voltage system. That is to say that a system based on 5S lipo turning 30,000rpms will draw less current than a 3S system turning 30,000rpms (all other things being equal). The higher voltage system will run much cooler.

I consider the 8XL and 9XL to be 4S motors. If you want to run 5S, I'd go to a 10XL.