I have seen this same thing in the Saab 340, Noticed that during cruise the torque was lower than normal. Couldn't figure out why, double checked cruise power charts and it was fine. looked at OAT reading and it also seemed to be reading warmer than normal. The FMS performance page showed ISA +30 which is not typical.
I was lucky to have another Saab flying same route a few thousand feet above at the same time and spoke to them to get their OAT reading for their altitude and confirmed that our OAT was reading warm. In the end there was a small blockage in the TAT probe which was giving us incorrect readings.