I honestly do like that idea.
The upgrading of summoners is really just something you have to do once, and then you can benefit from that forever. The loss of half of you 15% will hurt, but if it means you're winning more, then you'll be earning more SPS per battle... I have no idea what the ROI of such a move would be, but I have to imagine upgraded summoners would be better (and more fun) in the long run.