I am in the same boat than you, 99% of my riding is on TR with a "dumb" trainer and wondering if a powermeter would help me in any way. I always felt that the "virtual power" number might not be accurate but are consistent and that the accuracy of the powermeter would be dismissed by the inaccuracy of the dumb trainer/tire/tire pressure combo. The argument that it would be useful for pacing on HIM it only appears to be true to me if you train with it outdoors too, i don't think ( and I could very well be wrong) that whatever numbers a powermeter would give me from a dumb trainer could be transferred to a real ride outside? I guess I am not helping with your decision, just my 2 cents, sorry.
In my n=1 personal experience, the PM didn't make me significantly faster after coming from TR+dumb trainer on virtualpower (was on a reliable Cycleops Fluid2). I was pushing pretty hard on workouts prior to the PM, so even on a great TR super hard block I was lucky to get even 5 watts added after killing myself on a lot of the workouts.
Still, at $500 for an Assioma Uno, I think it's a great investment, as they are still very valuble for outdoor workout tracking, race execution AND analysis, and all that good stuff mentioned above, even if it doesn't make you a ton faster.
I have lots of tech now, but honestly, in my 12 years of tri, having done quite a few programs from TR superstructure, to freeform 'no-plan', I think it all comes down to how much hard training you can put in with good recovery. For sure, a powermeter or even a structured training program won't give you a warp boost in performance unless you were slacking off pretty good to begin with.