Lets say the Stages Powermeter is quite accurate for the left leg which it no doubt is. Lets also say that the Sky team has done correlation testing with say a power tap rear wheel that has been passed along to team member's bikes for baseline correlation testing against Stages such that riders can deduce what the left/right leg bias is...if any.

With above knowledge, then there is no reason why the Stages PM wouldn't be useful. At the end of the day once a baseline is developed...and it varies for every rider including top rides like on the Sky team, then it is the relative increment of power that matters...not any absolute number. I believe above is the reason why the Sky team can and does use the Stages PM effectively. For the average guy...lets assume a left/right leg power difference of 5%...as most will have a dominant leg. Same thing. Chances are this leg bias or disparity won't change even with training but the relative or normalized power of the rider will change over time with training and improvement.

Agreed.

Having ridden with the Vector pedals for a couple months, I've been surprised to find that any L-R imbalance tended to be left dominant by up to 4%. It seems like I am more likely to see 50/50 or slightly right dominant on easy/moderate pace rides. This is odd to me because my left leg is weaker than my right - my left hamstring is a lot smaller due to ACL surgery taking some of the tendon. Garmin has stated that their testing has shown L-R imbalance to be a dynamic variability, in that it's not really consistent. I think this could explain why some power meter tests show the Stages reading a little higher on some tests, and a little lower on others compared to powertap, srm etc.

In any case, part of the reason I invested in the Vector was the assumption that my left leg was substantially weaker, and therefore a Stages would rob me of precious e-wang. This turned out to be false, and I would probably be seeing slightly higher numbers with a Stages. Whatever. Testing shows that the Stages is close enough and consistent enough that is doesn't matter a ton to most folks [i.e. Sky]. I am still pretty stoked on the Vector, but all my team mates are getting Stages and I can't blame them.

I think you are just short of realizing the important discovery you are sitting on:

ACL SURGERY INCREASES POWER BY 4%.

Maybe more, as your left leg may have actually been weaker to start with. I just scheduled myself a double for next week.

Yeah, this is actually pretty consistent with the observation that cycling is mostly an aerobically-limited sport and that at FTP we're typically way (way way) below maximum pedal force or crank torque. I have a friend who's a coach who unfortunately had a mid-femoral amputation of one of his legs a few years ago. He had power records for several years prior to his accident and, of course, since his recovery. It took about 3 years post amputation for his FTP to match his pre-amputation level but his top end sprint sucks now. This is also consistent with the observation that pedaling technique and oval chainrings appear to only affect short-duration power (if at all) but not FTP. So this is why I've been saying that unless you do things like QA (or if you do things that actually require accuracy across a wide range of powers, or if you do sprint training, or drag testing), and most people don't do that kind of stuff, the Stages ought to be okay.

One thing I've wondered: I have a 15mm leg length discrepancy (as measured by the radiologist, once my fitter pointed it out). Before I got that corrected on the big I had given myself a nice cace of tendinitis. Anyway, I already know my legs are rather different from each other. This makes me think that a single leg solution is not ideal for me. And perhaps a solution with L/R balance could help my physical therapist (who is fortunately a cyclist) shape what he has me do.

Stages is admittedly half the price of other options. I could get a Quarq Elsa R for about the same price as the InfoCrank setup (although the InfoCrank comes with chainrings). $1700 is way more than $800...but I wonder if it would be more useful than just measuring one leg (when I already know it's the weaker leg).

Maybe I'm missing something, but won't 1 minute at 90 RPM's generate the same watts as 1 minute at 90 rpm's on the same gear with any left/right imbalance ???

Yes. But the trick is in measuring it correctly.