If you wanted to go with 54W bulbs you would need a t5h0 unit something like this.
http://www.bigalsonline.com/BigAlsUS...000kfreshwater
I personally find t5h0 units run very very hot, and tend to use two sets of regular t5 units running four 28w for 112 watts compared to two 54w for 108 watts. As then I get more even light coverage and they run much much cooler.
Plus watts to gallon rating was based of t12 bulbs so you have to take into consideration the lumen to watt ratio of bulbs when figuring out watts per gallon.
Say if you had a 55g tank with 2 sets of coralife bulbs for 4 28 watt bulbs or 112 watts, that would like 2.24 watts per gallon, but its not as regular t5 bulbs get multiplied by 1.37 compared to a t12. So that 112 watts is actually 153.44 watts of t12 rated wattage as t5s are (1.37) as effective lumen to watt. Bringing your watts per gallon to 2.78 watts per gallon.
If you took the same 55g tank with 1 set of t5ho with two 54watt bulbs for 108watts you would break down like this.
108 x 1.22 (1.22 is the lumen to watt rating for t5ho compared to a t12)for 131.76 watts of power divided by the 55 gallons for a rating of 2.4 watts per gallon.
So a pair of T5 units will give you much more even lighting with lower heat for 2.78 watts per gallon for just under $100.
A single T5ho will only strongly light the front or back half of your tank and produce more heat and give you 2.4 watts per gallon for just over $100