Couldn't you use something like a Stirling engine to create more electricity from the excess heat? You might have to have an external radiator to create a heat gradient with a closed loop of liquid but it certainly seems possible.
In theory, the best you could do is (T_hot - T_cold) / T_hot, or (430 - 210) / 430 ~= 50% , using some approximate values for fin root temperature and Martian climate. To exceed 10% of the theoretical optimum with something that contains no moving parts is pretty impressive, IMO.