Somebody with better physiology knowledge give the following a thumbs up its true, thumbs down you're full of it evaluation: If it's hot you have more blood diverted to vessels near the surface than when it's cool, so for the same oxygen delivery to muscles there's a greater volume of blood that needs to be pumped. If the above is true, then I'd say that, if you're trying to induce training responses to move more blood, then the training effect in heat is higher for the same power. If you're trying to generate different physiological changes, then the training load may not be any higher than if you did the same power at a cooler temperature. My conclusion would be that tss is an estimate and is reasonably robust to being close rather than exact, so I would never worry about it.
And how's that for holding contradicting opinions with myself.