I don't exactly have a great track record steering you in the right direction, but I'll share anyway, for what it's worth. It might seem counterintuitive, but the hotter the water is coming out of the returns, the
less efficient the heating is, believe it or not. The temperature of the water at the returns coming from a properly optimized heater should feel just slightly warmer than the pool. 12-15° is not that. It should be more like a few degrees. The other clue is when you mentioned 1500 RPM. That's probably low. This has to do with the physics of heating a pool (which I only barely understand). A large volume of slightly warmer water will heat a pool better, and faster, than a small volume of very hot water. If your flow rate is low (like it might be at 1500 RPM), then the water in the solar panels has more time to heat. That's why it feels warmer at the return. But it's the
volume of warm water that better heats a pool, not the temperature. So you up the RPMs, which ups the flow rate, which will
lower the output temperature, but warm your pool better because there's more water moving. It's something to do with physics of heat exchange. Maybe
@mas985 can explain it better.
I need to run my pump at about 2200 RPM to get the optimal flow through my panels. I determined what the optimal flow should be based on the manufacturer's recommendation (I figure they would know). I verified that 2200RPM produces the correct flow rate by installing a flow meter (FlowVis) and adjusting my variable speed pump's RPM until the FlowVis indicated the correct rate. Which for my eight panels is 40 GPM (gallons per minute).
That all said, if your pool is warm and it feels good, then none of that matters!
I was told by the solar installers that sold me my panels that solar water heating panels are not anywhere near as sensitive about their angle to the sun then electricity producing PV panes are. The latter have to be pointed directly at the sun for maximum output. But the former just needs to be in the sun, not necessarily pointing just so. Your panels are probably just fine where they are. Mine face North! I saved the South side for my PV panels...