Well for me, the heads up LED vs. T5 even would be a neat experiment...even if it's already been done, one of my personal concerns right now is that while I'm getting some really nice growth from my LEDs, the coloration I'm seeing is truly different (i.e. my Sour Apple Birdsnest is not green and orange, but is lavender).
Such a test is relatively easy to set up (2 linked aquariums, lit independently, links water volume and thus all chemistry).
The pitfalls I see might include how you pick only fixtures to compare. I.e. what you'd really want to find is either a) fixtures that consume the same wattage (so you can look at this from an efficiency standpoint) or b) fixtures that output the same measured PAR levels, so you can look at this from a "everything is the same, yet the differences are dramatic/not dramatic" standpoint. Time could also be a factor - do you have enough time to get the growth and measurements and results you'd need to make any definitive answers.
Obviously, the meaning of the results would be limited to a comparison between two individual lighting models. I.e. I don't think you could go so far as to say "definitively, LEDS are not as good" since you're only testing a single unit against a single unit (assuming that's how you set up the experiment).
The development of a fish IQ test? Interesting and a lot of work has been done in that realm before...but I'd want to know a lot more here...