re: Scenario: I want to get a 6+ but company only approved iphone 6's

You should really be careful about this since most likely the company considers that phone property. If you get rid of it without approval, there is a very real chance it could be considered stealing.

Also if they don't happen to notice it, which most corporate accounts will flag new devices, if you are let go or such soon you would have to turn in the 6+ and be out the extra money you paid.

I'd be very careful trying to pull any of these workarounds. Go talk to whoever is in charge of it and be up front with what you want. Dodging IT staff and messing with company property is almost never a good idea.