To add to this, let me say something which I admittedly don't know for a fact with regard to TRMs but from general knowledge regarding such parts as computer PSUs. Usage of GaN would have an effect on this, though I cannot be sure as to the extent & the specifics.
Besides what I've already stated in post #3257, there may be several reasons why you'd want to put TRMs rated for higher wattage on your radar, even if your cooling system is obviously not capable of dealing with a full array operating at peak transmitting power.
Take the example of a 20W-rated TRM. While 20W may be the peak transmitted power, the peak efficiency is likely to be achieved at somewhere between 50-80% of the rated power depending on component design. Efficiency meaning how much power you have to input in order to get a set output. Closer to the peak rating, the efficiency drops off drastically.
So getting a 20W TRM would be a good way to ensure that operation at somewhere around 10-15W is performed with superb efficienc