I'm thinking about getting one as well and wondering if my boy (smaller 8 year old) could ride it too on low settings.
Do you think that would do it or should I look at the 20?
Someone correct me if I'm wrong but the cells have the same capacity but because the MXR cells will let you draw power faster, you usually end up with less range on paper. Not really noticeable in practice.
I highly recommend against this: the caliper is already thin in that area compared to stock units and grinding more metal off will weaken the wall significantly.
You might even go through if you're not careful.