maximizing light may be helped with positioning, but more important is that your room is "tight". by tight i mean that it needs to be close to the expected footprint of the grow, so that there is less square footage for the light to have to travel. Smaller things are using really good material like mylar, floylon, or what you think is in your budget. i came across this link on reflective shit
http://www.420magazine.com/forums/how-grow-marijuana/71077-what-reflective-materials-can-i-use-my-grow-room.html . also you can put a reflective cap over your bucketts on your plants but be sure not to make it permenant so you can water and also it will slow down evaporation from the pot. if you think this will cause dampening off (rotting at the stem) then just raise up the cap a little so air can get down to the soil. as for how exactly you need to put your lights, i think on top will work great as long as you have a "tight room" that is reflected properly, the t5's give off a good amount of light but hey, the more the merrier, just make sure you remember that your getting less % from each light for how much bigger your room is and how you keep everything reflective.
one more side note, that a lot of people don't mention, but might know. There is dust in the air , whether you like it or not. this dust is getting on your plants, on the walls , and even the fan blades on your fans. It's a great idea in my opinion to add a filter onto the intake (maybe a HEPA, or just a course filter) to make it so no dust is entering your grow. The dust on your walls will reduce the reflectiveness , which is why its good to have something cleanable, that mylar emergency blanket shit is hard to clean, you're almost better off not cleaning it and just replacing it, but i have no experience with the higher mil mylar material, so am not sure how that is. dust also goes on the plant, getting on the leaves, and making it less productive ( i have no links to back this but just think about it, it makes sense, dust will allow less light, less air, and other shit i probably cant think of) so yeah know that its not all about the position of the lights, but more about how you design your room. I have a nice tight room and just keep it as close to the top of the canopy without burning it. the lower growth stays healthy. As for an all t-5 grow i would do side lighting but I use an hps for flower so i'm not worried about penetration.
