Okay, back up once. The Icecap retros are more expensive because they are using the Icecap 660 ballasts that overdrive the bulbs to 80watts each. What can do that makes more sense is to get the Tek retrofit, with normal spec T5 ballasts, and call Greg at Geek to have him swap out the Tek reflectors for the better Icecap SLRs. They are a much nicer reflector than the Tek, and only cost you a few $$ more per bulb... so much less money than the 100% icecap retro.
These are retros though, and so if you dont want to DIY, then retrofits are out. Fauna-Marins, ATIs, and possibly the Teks. I do have one warning about the Teks though... you get what you pay for. The reflectors are corrosion prone, and the passive ventilation isnt enough to keep the bulbs as cool as they should be. The heat buildup kills anywhere from 10-20% of the bulb's output, and the acrylic sheild that protects the reflectors makes it even worse. The best solution I have come up with is to use a cross-flow fan attached to the Tek on one end that blows air across the bulbs across the bottom. The acrylic sheild can help in this case then... if you take one end, and notch out about 1/2" in width, and 2" in length at the corners, you can then heat-bend the acrylic so it has a lip and opening at the end where the fan is... so the fan output gets blown up into the reflectors, across the entire bulb, and out at the other end. This also keeps the reflectors protected from splashing, so its a win-win. But its another cost and time issue. FWIW, the Fauna units might be a bit more, but worth it. They use coated reflectors that you can easily snap off for cleaning, and they dont corrode. They have better ballasts and active cooling built in, as well as nice waterproof endcaps. The FM doesnt come in a 8x54watt unit, but the better reflectors and tech that is built in makes the 6x54watt unit just as bright... so its a electricity savings for one. The 6 bulb unit is $579... but includes the bulbs, so its really only $70 more than the Tek (and the extras for the Tek if you do the ducting mod or add fans makes that amount trivial). If you want to stick with 8x54, you can mount 4x54watt units together, right against each other... they are $449 each though... $900 total. But you intend on using two of the 8x54watt fixtures somehow... perhaps three of the 6 bulb units from fauna would do... or just two of the 6 bulb.