DAMNFirst of all, if you buy a 256Gb and fill it up with 256Gb, you need to buy a bigger one (which isn't available).
Second if you only get 20MBPS you have a faulty AP, a whole lot of interference and/or need to start using 802.11AC at 5GHz.
Third, if you fill it up with say 128Gb, it will take a bit over an hour to get that onto the iPad Pro, after that what is the problem? Adding 20Gb costs me almost no time. A little speed test gets me well over 500Mbps on the Internet and in my local network even much better.
Fourth point, if the USB3 controller is your only problem, tell me how much of the time is your iPad connected to a USB port? 1% of the time?
Last point, it usually useless to sell the current generation for the next generation iDevice unless there is a really good reason. With the iPad Pro 9.7 there are several reasons: Pencil, smart connector/keyboard, 256Gb or the new display.
If all of these don't convince you because of a USB 2 controller, then just buy another Air 2.
I believe the kids I play Xbox live with have a word for this...OWNED