I use a PM with TR, but I thought that the virtual power was based on speed and cadence... meaning, cadence is the only way it truly knows how to estimate the torque output... I could be wrong, but as the OP posted, he could be on a very easy gear spinning very fast, but if TR doesn't know that his spin, it can't calculate power.
At the end of the day OP, it is all relevant and will reamin consistent and you will compare your rides against your own #'s so you will improve