700X20c

I don't think there's an easy way to answer the question. Wider tires are more comfortable, which is not directly related to speed (although could have an impact on endurance and therefore speed over long distances - if you have to come out of aero and stretch out because you're uncomfortable that's going to cost you time), but they also have lower rolling resistance, which is directly related to speed. So given a 20mm rim, running a 23mm tire will not be as aero as running a 20mm, but the combination of lower rolling resistance coupled with decreased fatigue over long distances *could* negate the aero losses. Then again, it might not. There's no way to say for sure.

You could split the difference and go with something like an Attack/Force combo - keep the narrow tire for the front where the aero difference is more significant, and have the wider tire on the back where aero isn't a big deal, but the impact on comfort might be more pronounced.

I think you first of all have to make up your mind in the weight vs aerodynamics choice. I assume you bought the Flo wheels because they are more aero than the Bontragers, and you will probably mount 23mm GP 4000s on them as they are the most aero on them (the GPs have been windtunnel tested with Flo rims specifically). Then you have your self claimed climber wheel, which is lighter, so then aerodynamics apparently aren't important, and if they aren't thenI guess you can also mount the 23's on that rim. If you really do care about the aerodynamics, then you'd probably not ride the climbers wheel anyway because aerodynamics trump weight unless you're going for some high alpine climbs, at which point I'd still choose the 23s because then you're at such a low speed that aerodynamics aren't important, and you might as well go the wider tires. I really cannot come up with a situation where the 20s would win out. Don't forget you have to put more PSI in your 20s than your 23s.

Thanks guys. This all makes sense.