using a 300bar cylinder will work as long as you have din 300bar fitting and the fill station near you does for filling it. They are a lot heavier though so should take some weight off.
Erm the ally BP swapped for S/S is a good swap, and would say that my gf used a 15ltr and she is pretty small, it worked fine for her.
Even if you don't need the extra gas its always good and the added weight is quite a lot. Does anyone know how much more negative a 15 is compare to a 12?