Not necessarily the case. Depending on the compression ratio of the machine (turbine in this case) you can compress hot air at a smaller rate than cold air and yield the same density.
But then again a model jet engine does not have the capacity of changing compression rates.

Relative humidity is more of a factor than temperature for a jet engine. the more O2 you can get to the expansion stage, the more thrust. Viceversa, more H20 means less O2 meaning less Thrust~equivalent to the leaning-reachening of a reciprocal engine when climbing/descending.

So: cool air + high humidity thrust can be lower than hot dry air thrust. Just a thought on Thermo I.

David