The best fixtures (or DIY retrofits) all include individual reflectors (FYI, IceCap SLR reflectors are the best in terms of punching light down) and active cooling via fans (very important, especially if you are overdriving--but still necessary normally, as it gives you a 20% light boost). Lastly, it boils down to bulb choice. Some lamps can have literally double (or more) light (in PAR) than other tubes--for this purpose, ATI, GE, and UVL bulbs are the better ones. Bulb choice is a matter of taste in the end, though, but the former factors are the most important in any fixture or retrofit kit.