I'd be inclined to clean your battery and ground connections thoroughly and double check that you have heavy enough cables, 2/0 is a good size.
With good connections and cables 6V systems start well. An 8V battery might just mask the underlying problem.
If you wanted more voltage I'd go ahead and swap to 12V, change bulbs, alternator/generator and regulator, and don't worry about the starter, they tolerate the higher voltage quite well.
