I think Steve pretty much summed it up. Just to add to it, my PAR reading on the T5 were with the old bulbs so yes, the reading would have been higher with newer bulbs. Couldn't do that as I just recently borrowed a PAR meter for this testing and had no way to test when my bulbs were new. And I wasn't about to spend $250+ on new bulbs at this point
I would love to see others posting PAR numbers from their fixtures so we can do comparisons.
And for the record, I wasn't making any claims as to comparisons with 250w or 400W MH as I haven't done any PAR readings from those but a guess as to where my light output would end up. If anybody else has let's see the numbers as it would be interesting to compare.
But what I do know is that with the LEDs lower down, at about 7 to 8 inches above the water I was seeing just over 500 PAR in the upper part of the tank and 300 just above the sand bed of the tank about 18 inches down. That's pretty good from my understanding. I didn't test with the fixture even closer to the water but if I lowered it to where I had the T5s then I suspect Steve is correct and PAR would be 600 to 700 in the upper part of the tank. Maybe I'll try it tomorrow and see. The reason I raised the fixture for now is that I would burn all my corals switching them from say 300 PAR to 500+ PAR. They need to be acclimated and I plan to slowly lower the fixture every few days to acclimate them to the higher light.
Give this article a read:
http://www.advancedaquarist.com/2010/2/aafeature2
It shows comparisons in air, not water but it's a start. Also you have to note that the LED fixtures used in the article are not the best. One is a PFO Solaris which used older LEDs that do not have the same output as the newer generations of emitters and the other is the Eco-Lamps one that under drives their LEDS at just over 1 watt instead of 3 watts. Even so they compared well to a high end T5 (about equal performance) and a 250W MH (a little less but not by much; about 15 to 16% less output). I guarantee you my fixture will outperform both the Solaris and the Eco-Lamps as I used very good Cree emitters driven at a full 3W and 1000mA for the whites and about 900mA for the blues.
Even so Dr. Joshi states that most Acropora and light loving corals will thrive at PAR levels of 300 to 400 which this fixture can provide throughout my tank if I wish. That's good enough for me. In a 30" deep tank for example you could keep high light SPS in the top 2/3 of the tank. I don't have a deeper tank to test on but I suspect PAR would still be close to 200 by 30" down. That's pretty good performance in my book.