The bankers can still use other banks to store their gold. But I do not think that this formula will create this necessity. A level 4 Bank already will allow the banker to save 35M - without the need of any bank operation. Isn't this enough? A level 20 Bank will allow 115M extra. The best I could think right now, it to make it level * 6.75M instead of 5M, just to make the max a nice and round 150M at level 20.
We do not really want to PUSH you to upgrade. In contrary; we want you to upgrade slowly. We just want to give you reasons to upgrade. If there are not enough reasons to upgrade, then there was no reason to put 20 levels in the bank.
For the Goldsmith, so far, it is working excellently, despite what some people may think. Be warned for goldsmiths though; growing too large too fast might bring you to a point where supply is greater than demand. Then, you will not be able to sell. Try to control your exposure to debt if you have took a loan taking this in mind.
Regarding the other perk for banks, 1 turn less for level 10 and 2 turns less at level 20 for ALL transactions, I think you all agree?