That's basically what the sun does. lights from a single point. There will be shaded areas, but that really shouldn't hurt anything....but...that's why there are so many flavors of ice cream
You might look into a retrofit kit to mount into a hood if you have one. You could stagger the lamps and this would light up the tank more evenly. If not, I wouldn't be concerned about overlighting with a 6 x 80 watt fixture. You see, T5 lamps emit light the whole 4 or 5 feet, and not brighter from a point like halides or LED
does, so intensity won't be much more at a certain point, but spread out.
In other words, the 80 watts are needed to power the entire 5' lamp, it's not brighter than the 54 watt lamp right in the middle. If it is, it's not by much, and you can't tell with the naked eye.