You may have figured it out already, but 30W 802.3at input on the 7782 should be enough to have 15W 802.3af on the second ethernet port, and you can use an ethernet switch or a 30W 802.3at injector for that.
It could be that the R750 is running on a different channel and/ or channel width from the router, due to interference and/ or settings, limiting the current performance. Testing them with the same settings should create a more level "playing field"
As far as I know, the controller and AP will bring the power down to the allowed levels for your area when you have selected the right country when you configured the system. So you do not need to adjust the power level yourself as long as you do not...