I also replaced my Mk 25's with Mk 17's. For extreme cold water diving the Mk 17 is the best first stage available anywhere.
The flow rate of 177 scfm is more than adequate for any rec or tec requirement and the air delivery is first rate. I use very easy breathing D400's on mine. I have dove with a Mk 25 on one post and a Mk 17 on the other and I can detect no difference between the MK 25 and MK 17 first stages at depths well below recrational limtis with the D400 second stage.
In terms of hose routing I prefer the MK 17's for doubles use to even the Mk 20/Mk 25. If you angle the first stages about 45 degrees and cross the hoses over to the opposite side, you get very simple and clean hose routing.