Ziggy the hobbyist is allowed to disagree with Dr. Sanjay Joshi, PhD (Industrial Engineering) and his methodologies.
If you read some of his many articles (which obviously you haven't), you will see water has an across the board affect. It does not affect 1 reflector performance more than another. So if water prevents, for example, 10% of the possible light penetration in 6" of water, it's 10% for every reflector. What would the benefit be testing under water as far as the differences in performance...which is what his tests are all about. The difference in performance.
This is all mentioned in his articles if you take the time to actually read them and not just look at the pics and graphs.
I find Sanjay's work an invaluable tool in selecting lighting. I can look at 2 bulbs and compare their output and choose what I want and know how that bulb performs. I can then take that and apply it to reflector performance. If the bulb I choose gets 75 PPFD per Sanjay's test (which are done with no reflector), I can trust that the performance of that bulb will react the same as the results in the reflector tests. If it gets 100 PPFD with a parallel reflector, I believe it will get 300 PPFD with a LumenArc, per Sanjay's test results. You just have to understand, these are all tests done to show the differences in performance. Real numbers in a tank at 6" deep may only be 50 PPFD and 150 PPFD, but the ratio (3:1) does not change.
When I was building my system, I asked Sanjay for recommendations on my lighting including what kind of reflectors to use. In fact, I think way back in this thread I mentioned I asked him about my lighting. I think I'll take the advice of Dr. Joshi.
BTW...my bulbs are 12" above the water.