Using a single 420GTS here with a DDC pump and two blocks - it's not really that restrictive compared to the blocks, and my pump has no problem inducing a sizeable vortex in my res when at full speed. My fans are typically in the 600-900rpm range with max CPU & GPU (hot spot) temps under 66°C, even after long benchmarking sessions (OC Vega64 and OC R7).
While a D5 has a lower pressure capability than DDC, I don't think the GTS rads would be a problem; in the worst case you'll just be running the pump at a slightly higher rpm than if you'd used a DDC. The D5 is meant to be quieter though, so I think it evens out.