Max level of floor upgrade?
No, there seems to be no limit to the number of upgrades to a floor. However, you have to decide if its worth upgrading up that high. The only obvious benefit of getting a lot of upgrades is if you get a big spender who'll buy up all the stock. That's when you have a nice bux->coins exchange ratio since you can spend 35 bucks to restock a floor and sell a few hundred thousand coins worth of items.