That's pretty much what I was recommended by the excellent customer service at Aquacomputer as well. Thanks a lot for the input!
Adding another radiator will help alot. For my 720xt i purchased the high flow kit AC sells to boost flow (i wanted approx 60 l/h minimum). It worked great installed about 1 meter from the base of the 720. I have the 720 on the floor, and the first water block (cpu) is about 1.2m above the main pump, both pumps are pushing "uphill". Adding a DCC pump helped raise the coolant and added a bit of redundancy in this setup.