Some fixtures effect the output but there isn't too much data on it. The fixture wattage must match the bulb wattage or it won't power up fuly and that can impact the output as well as longevity of the bulb. It's recomended for efficiency and the life of bulb to use a electronic ballast vs a magnetic one.
More than anything a good reflector make a big difference. A reflector really helps boost output, by about double or more depending on the shape and material.
I would get a T5 fixture over a T8 any day and most applications will benifit from a reflector so you can mount the light further away.