[quote=GillenvyI’m drawn to Rebreather’s in part because I hate worrying about how fast my gas is being used up when I’m in a current on OC…there are plenty of times when I would just assume kick harder, stay warmer and extend the dive…if only I had more gas.
In your oppinion does a particular unit hold up to higher respiration rates in cold water better than another?[/quote]
That just about sums up why I love my rebreathers so much, huff and puff against a current in a cave or swimming to a ascent / descent line….just keep breathing and enjoy the dive. 
Regarding CO2 and breakthrough, I have had experiences in cold and warm water on the inspiration and I have read that over-breathing this unit is not possible. Well I would put the person who stated this in my fins when the feeling hits you and the only resolve is numerous flushes. I have never experienced the same feeling of over-breathing or CO2 excess in the Meg, and I have not had my KISS units in heavy current situation yet. I have only had the Optima in the Devils system a couple times in good current, but warm water and it seems to breath quite nicely – not enough time to make good comments yet.