im not too sure bout battery suppliers, but should be able to help with the rest.
Firstly you current batteries, have around 8Ahrs, judging from your suplied range and top speed data.
The milage will just scale accordingly with the increased ahrs.
Bigger motor = higher speed = less total milage.
Total battery weight varys bike to bike, keep adding them until it starts to feel heavy (or greatly exceeds the original weight) would be my advice.