The only time I have visually seen an improvement is with my over driven bulbs. After the change out I can tell the colors look better, so from a visual perespective it's immediately noticed. Within two weeks I always get a color and growth spurt. With standard driven bulbs I don't see this but that's with the Sunpower and the active cooling. I only change out the over driven bulbs at 5-6 months. Sunpower I go at least 12.
You can definitely get over 12 months in a Sunpower or similar. BRS test had no cooling for the bulbs(they took out the plexi shied) and they got that 12-14 month data, so it's probably reasonable.
The par loss is not an issue but spectrum is and their test on the spectrum shift wasn't much. So, based on my experience and their testing I would go at least 12 months.
On this---
"Also, If I end up going with a 96x48x24 peninsula then how many T5's would I need to cover that foot print? Would 6' bulbs be a good choice for an 8 foot tank?"
No on the 6' bulbs.........you would need 4 banks of 6x54w at minimum. So four 48" 6x54w Sumpowers or similar. If you stay with that footprint on your new tank I would look for better options mainly due to the future of bulb availability. You don't want a deep sunken cost that in 5 years you have to completely overhaul your lighting.
The issue is you know MH and T5 are going to work for the acros. With the LED or some combo of LED and T5 you are going to have to experiment. I've seen some impressive tanks that had a mix of MH and LED.
LED alone, I can't get past all the shadowing and darkness.
I haven't seen anything in person that convinces that LED only is as good as a combo with T5..............it's always better if t5 is the workhorse and LED is supplementation as well. The LED as the workhorse is ok but not as good.
From a lighting perspective I think 48" width tank is going to be a headache. I think I would stick to 36" max for a new tank. It also will be a bear to maintain.........even with a peninsula design. Don't let my hangups stop you from having what you want. If this is your dream tank go for it

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On a much smaller scale I just bought a 60x24x24 tank and I'm struggling with the same issues. If I had gone to a 48" tank I'd be more inclined to stay T5 with a few LED strips because I have all the lights currently. With this 60" length I have to buy new lights. I don't want to put up a 48" fixture and then buy something else to fill that last 12".
I'm not discounting LED only but I would have to do more research as far as coverage and number of units. I like the Mitras 7206 & Reefi duos as they have reflectors instead of lenses. This makes it an easier transition from traditional lighting as you don't have the magnifying glass bleaching effect as well as no concern for burned out acrylic lenses.
I am also considering the Orphek icons as they have now gone to glass lenses but transition is still a concern.
T5 only it's an 6 or 8x60" new fixture and hope I can still get bulbs for 10 years. This seems the easiest and what I know I'd be happy with.
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It's never an easy decision but just go with your gut and what you feel you're willing to work with. You may want to check out Reebum's system. It's 36" wide with only MItras. He has a recent video showing growth and color and it looks good.