Mostly, I just wondered about LEDs losing lumins over time. I know that all lights fade, but I don't believe T5s fade all that much. It's good to know that LEDs will also heat up the tank a bit, as I'm currently using the T5s to heat up the tank a bit during the day, along with an increase in the heat, to give me a ~20 change for my plants over the course of day-night. During the summer, I hope that the cool night air (no AC here, unfortunately), and the light during the day will have the same effect, since the tanks are no longer exposed to the sun.
I'll definitely consider getting some LEDs in the future, although I kinda want to wait for the price to come down a bit, since I can get a 4 bulb T5 unit with bulbs for ~$50. Definitely keep us updated on the experiment. And, maybe, you should write a new sticky for lighting, especially since the old one, if I recall correctly, mentions T12s, which are being phased out and really shouldn't be used.
All lights fade over time, from my reading the common #s for LEDs is they maintain 70% of there brightness over the listed life of the bulb. So if its a 50K bulb it starts at 100% and would be 85% at 25K hours and 70% around 50K then a much more drastic drop off until they burn out. Now all those technical data sheets quickly get over my head and hopefully someone with more knowledge will chime in. I do know from experience that T5s have a drastic drop in light over the first 6-8 months, if you have a large number of T5s and only swap out the bulbs when they burn out you may never notice this. But I have a few tanks with 1-2 T5(s) over them and I clearly see a difference when I change a burned out bulb with a new one. The single bulb tanks I have to be very careful as some of the more sensitive orchids will throw a huge fit if I change their light that much in a day.
The main issue with using LEDs for heating is that you need a heatsink for them to work properly and that means all that heat that should be heading toward your plants is actually getting stored/dissipated above your LEDs and if my reading is correct the more heat the more life of your LED your burning up. You could probably work something out with fans and whatnot but I'm still of the opinion that operating them that hot is not the best solution. I think when you get into the math you want lots of LEDs running at lower power, I'll try to explain why later when I get to the real costs of the T5s. I do really love T5s they were a massive improvement over T8/12s and allowed us to grow a much larger range of stuff but I fear they are misleading people and costing them a ton of money they are not even realizing they are paying.
As for price, IF all the information about LEDs is half correct your much better off replacing all your lighting with them RIGHT NOW! That said I can not verify even half of that information is correct
So far 1 LED powered at around 1W per 6" (8 for a 48" area) is more light than a single 48" T5. The LED are spotlight and the T5 are bar lights so that must be considered. (I come to this conclusion from testing with the moonlight kits and plants moved from tanks with old/new T5 bulbs and the coloration differences once moved. Light measuring meters are mostly useless (they only measure visible spectrums not the spectrums the plants are using and red/blue spectrum would grow plants well but the meters would read as low light or the ones that would work are very expensive and impractical for us to own))
So 1.25W*8=10W max power for the LEDs vs 54W for the T5. (These #s are from the technical specs and were not measured so they may or may not be accurate) Over a year 10 hours a day it would be 36.5 KWhrs for the LED and 197.1 KWhrs for the T5. If I recall correctly its $.104 per KWhr for me, or atleast close to that. So it would be $3.80 to run the LED for a year and $20.50 to run the T5 for a year. During the FIRST year it would cost an additional $16.70 PER T5 to operate and I believe we have cheap power down here.
Running costs for that 4 bulb T5 would be $82 a year. If you replace the bulbs on burnout you would need to replace them 2.5 times before the LEDs needed replacement.
Running costs for 4 rows of LEDs would be $15.2 a year. Assuming the bulb life is correct nothing would need replacement.
Over the 13.7 year life of the LEDs if would cost $208.24 running costs before you needed to replace probably everything. Rough guess of $350-400 initial costs. I believe you could run 3 rows and it would still be more light than the 4 T5, but I have not set anything up to see. If that belief is true than the costs would be $156.18 with a guess of setup at $260-300.
Over the same time frame if would cost $1,123.4 and you would need to replace the bulbs 1.5 times. You "might" be able to find T5 bulbs in 14 years! $50 initial costs
Now you could get a real LED kit and run it about 35-45% power and use less LEDs but I have no way to compare the two.
For short term T5 is way better, once you hit a few years of use your wasting money assuming all this information out there is correct.
That 4 bulb fixture is an amazing deal, I paid close to that for each T5 fixture when I bought them years ago. All I can say is I've been amazed at the results I'm getting from such low power. I'm way more impressed with LEDs as a low power application than I am using LED to get massive dark colors off plants. Just thinking of all the plants I could grow with 1W of power is amazing.