Re: performance differences
There are only a handful of people on this forum who've owned two of the three boats you are considering. I am not aware of anyone that has actually owned all three but there are a few who have likely sailed on all three. So while you might get some user feedback from a few people with the experience that you are looking for, you are not likely to get a whole lot.
That said, I think you nailed it when you said "I'm assuming the sailing performance between these three hulls is small enough that it's likely to boil down to how new a boat I'm willing to pay for."
Given that a good SM will run you roughly ~$300-350k, a 54 ~$550-600k and a 55 ~$800k++ it would seem to me that the difference in sailing characteristics between the three will be far outweighed by the difference in the price you will pay. After all, these three are all blue water cruising boats designed to take you comfortably and safely across oceans.
SM #440 Cara
Deale, MD USA