The 10-15 deg rise is too much and the efficiency could be better. Maximum solar efficiency occurs with the lowest temperature differential between inlet and outlet. There is a happy tradeoff between temperature rise and flow. Every system is different and anything is better than nothing especially when it's free. I get about a 4 degree rise on a 4x20' mat heater at around a 15 GPM flow rate (very rough numbers from memory). I don't try and control the flow exactly but shoot for about 1/3 to the field heater, 1/3 to the mat, 1/3 to the returns.
Al