I currently have a 58cm road frame ( free but a tad big). I should probably be on a 56cm. I got my first tri bike in May and ended up with a 52cm QR Kilo. Anything in a 54cm I tried was too damn big. I am 5'11" by the way. Apparently I have a short torso and long legs. My friend who got me fit and setup at my LBS was shocked a 52cm fit so well. It is an extremely aggressive position for the shear fact that the short frame a small headtube and my seat has to be cranked way high for my long legs. I am slowly getting used to it. My back did not like it much at first but it is getting better.
I have heard there is a point where too aggressive can decrease power but I feel I had no better choice with my sizing. Now if you are purposely setting your bike up over aggressively trying to get out of the wind, you might be hurting power so much it hurts you more overall. This might be when it is time to get out a power meter and do some testing with positions and power output. If you got a bike that fits though, that's the best start. You can always play with adjusting the position on your current bike a little here and there to see if you can get more power out of it.
The only downside may be if you are doing really long distance and plan an IM. Those guys don't ride quite as aggressive because they are going to be on the bike for so damn long. If you are sticking with shorter races, no worries.