Basically, with the PRO-HD, the picture will be the same on the Android, you will not get full screen on it. The main difference will be using your computer (Mac/PC).
how much of a pita is it to upgrade a box and get it working?
The setup is not complicated, you just need to connect the Slingbox PRO-HD to your Set-top box and then run the Setup Assistant at the Slingbox location. For the Internet Viewing setup you might want to use the Network Assistant