I know that but it's not that easy to switch if you already have the stuff.. ;)

More sub-c cells won't fit and I'm not sure about 4/5 cells.
My old 4/5 pack (8 cells) seems to be longer than a sub-c stick-pack, as one would expect.
The battery holder is really optimized for 6 sub-c cells side by side.

At the moment I want to get the setup to work and maybe later switch to LiPo. This would make quite a difference in weight and/or runtime. 6s and a slower motor would help with the heat.
Not cheap and as durable though.