From what I can tell, the bank space works just like the ammo. Only difference is, in Raw data there's a line that says "Max Bank Slots" I think. Once you increase that, your bank will increase just like the ammo. It worked like that for me anyway. I set the black market upgrade to 15 & set the max bank slots to 30 & I had 30 slots in the bank. I'm sure I could go higher on the max bank slots & it will continue to increase.