BB74, I think Eric is right. I would expect that the major difference would be on the load that the boat can carry without affecting too much the performance. The 12.50 can probably carry almost the double of the load the 10.50 can carry. The extra boat mass will make it probably a little less nervous than the 10.50.
Regarding directional stability, when I have tested the Opium 39, the first thing the guy that went with me has done, still in the marina, was to lower the keel. It seemed odd to me and I asked why? He told me that the boat was very hard to steer with the keel up. It seems that the problem came with the territory (two rudders, swinging keel, large transom boats). I would not have many hopes that with the 12.50 it would be any better.
I think you're right on top again, Paulo.
With the swinging keel up, you not only get less resistance to windage abeam, you also miss the deepness of the keel as a central turning point. Therefore there is more slipping than turning, which is an issue one needs to get used to.
I certainly will have to, since 3.00m will be quite a lot of draft!
But I think this drawback can be acceptable, considering these boats are first designed for sailing, not for maneuvering.