Piglet,
T5 can be use to keep SPS. But i dun think it will make a lot of difference by using 80w t5 or 54w t5. Personally i dun think 80w ballast come cheap. 54w ballast on the other hand is easily avail. Imagine if one of your ballast fail. Where to get replacement? how long would you need to get another?
Can your sps wait?
I feel that by alternating the lenght of the 54w tubes, you can acheive a homogenous lighting in your 5 ft system. If you feel that the side is still going to look dark, you can also add in another 2 x 24w at the end of the tank perpendicular to the front of your tank. Meaning you have 8 x 54w t5 paralled to the front of your tank, and another 2 x 24w perpendicular to the front glass.
Just my 2 cents