Yes, you are susceptible to "weeping" of a t/p valve anywhere over 100 PSI. 120 PSI will probably weep even on a brand new valve. Remember that it is basically a spring, and at 150 PSI it would be full open.
You have a solar system, and that is a different animal. You definitely have to be designed for over temps and over pressures. I do not know if a simple XT is enough. I suggest you call your solar installer in to verify your system design.