The K15 and Ace are really very different in the way they feel stability wise. With the K15, you have that rounded hull so it's very canoe like and roll-y. However the balance is aided by the 30" width. The Ace has an almost completely flat bottom with some concave so it is not roll-y, it stays upright and then tips more suddenly. But in my experience, and being close to your height and weight, I feel the 25" Ace is perfectly stable. The lower standing position makes it more stable than most boards in the 27-28" bracket. The dugout thing is definitely an acquired taste, but as you are experienced with a K15, I don't imagine it would put you off.
In terms of stepping back, check out this video of Beau O'Brien as a good example of how many of us use limited movement inside the cockpit (so to speak) and rather than step all the way back, we drop a knee and lower our centre of gravity...
www.seabreeze.com.au/forums/Stand-Up-Paddle/Review/2014-Starboard-Ace-downwinder-Beau-OBrian/As for doing buoy turns, while some have mastered the kick turn on an Ace, if you ask me it's completely inefficient so I just tend to sweep turn on that board.