While you say your tank is special due to size, its only 2.5ft deep,your not breaking any new ground there. Surely even LED's at their current development state can provided the required light for growth at that depth.
The LED fixture we are using now is 140 watts. The PAR reading directly below the fixture (which is sitting directly on the tank) on the substrate is 50. We want at least 100 on the bottom.
The AI fixture at MACNA was suspended over a 24" cube. They claim the PAR values at the bottom of the tank in the outer margins are 100. If we were to go with the AI unit, just as an example, we would require 24 of them to cover the tank adequately. That's 1800 watts which is equal to 4.5 400 watt or 7.2 250 watt MHL fixtures. The initial cost of $14,000 - $15,000 negates years of energy savings. Having said that, cost isn't a significant concern here.
One of our concerns is cooling the LED fixtures so they don't burn out. It's one thing to run one or two over an open tank, but a closed cabinet with 24 of them is another story. While they don't radiate heat down, they do generate a lot of heat that rises up.
One feature I like about LED is that it is fully controllable. MHL is limited to dimming and actinics for bluer phases. The current LEDs on the market have the same shimmer as MHL, but they lack the rays of light that refract in the water. It's a personal aesthetic preference, but I find many LEDs on the market look clinical and artificial like T5s. Many people prefer the look of T5s so it's enturely subjective.
The fans are relatively quiet but you can hear them. In our case it's the only audible sound when you view the display tank and we only have two fans going on the one LED fixture. Even during 34c weather, the tank has not gone over 78f. We shut off the climate control system for the fish room for a few days and added the doors above the tank. The temperature went up to 82f but that's without the exhaust fan and chiller going.
One application where LEDs really "shine" is over retail coral tanks. LEDs show hidden colours as corals fluorece. T5 lighting was the retail light of choice but LED looks slick with a dramatically smaller footprint.