My limiting factor in my solar panels is how much pressure they can take without leaking. That's really it. At low speed, I can open them all the way, but at high speed just one turn of the valves is all they tolerate.

Still, on a good sunny day I can get 8-9 degrees increase. I was in this past Saturday in 82 deg water and 68 deg air temp, and that's in the NE part of the US.