Here's a picture of the rear mounted dual water bottle cage. I'm especially happy with the way my spare fit snugly beneath the cage arm between the seatpost and cages. It's not wider, nor does it hang any lower than the cages and therefore should not add any additional drag.
Last but not least is the AeroDrink that I used some last season and have being racing with all year this season. My friend Bob showed me a little trick of putting a bottle top over the big hole to prevent getting sloshed when you hit a bump in the road. It works nicely. It fits snugly but you can get it off and on with one hand to refill while riding.
For racing the IM, I'm thinking that the two in the rear will be feeder bottles with 3 hour mixtures (6 hours worth) of nutrition (Sustained Energy, Perpetum, Infinite ??) that I can mix in the aerobottle with water from aid stations to make a 1 hour mixture and the Razor will have something different for a little variation. I'm not worried about weight for the IM because the course is extremely flat. Less climbing than my 15 mile commute home from work in fact.
For training I guess I'll just fill them all up with 1 hour mixtures of whatever nutrition I go with and that'll be starting point for experimentation.