The more T5s you can fit over any tank, the better.
Covering the whole surface area of the tank is ideal.
Factors you need to apply for better performance and efficiency of the T5HOs is the quality of fixture and bulbs. Good reflector and cooling system for the bulbs like the ATI Sunpower will give you much better results than those cheap fixtures found on Amazon.
Using a good combo for what you will keep could add to that, logically.
In a nutshell: cover the entire tank, or most of it, get the best fixture you can afford and use quality bulbs.