so, i've found a t8 to t5 converter. if i change what would be the benefits - do the really pump out more lumens per watt?

Ive never used T8, but I haved used and still use T5's. I love my T5 light fixture and will never turn back. Unless I feel the need for the MH look. The reflectors for T5 bulbe wrap around the bulb and give off a great reflection. I think you get more use out of the entire bulb instead of just half.