Hi Ken, sorry for the late reply but as an owner of a recent Amel 55, I wanted to share a few thoughts with you. I have not sailed a A53 or A54, and I bet Joel's extensive summary of the differences is right on but I hope you will find my remarks useful nonetheless. I have sailed a lot of different boats over the last 50 years so I can put the performance of the A55 in perspective. I grew up sailing on my family's Arpege half tonner and Nicholson 38 yawl long keeler, have sailed extensively on a Moody 425 sloop and have owned most recently J105 and J44 race boats. I have sailed my A55 some 7,000 miles in total, from La Rochelle to Gibraltar, up the Spanish, French and Italian waters to the Aegean and back to Sicily. We've seen 35 knots on the nose West of Gibraltar, 45 knots in the Medicane Zorbas last year (downwind) and many many days of light wind sailing, with more than a fair share upwind. In all those conditions the boat has done great and any pounding has been minimal, particularly
as compared to the prior race boats I have owned or sailed on. Pounding is not a factor that I would be concerned about - it's a trade off for being able to make better headway into the wind in heavy air conditions than with a more traditionally shaped boat. The ample volume upfront makes the boat go over the waves rather than through them, which results in less hobby horsing but some pounding.
I bet a more important difference between these three designs is the light air upwind performance. If you look at the keel of an A55 and a A53 and A54, you will notice dramatic differences. The keel of the 53 and the A54 is very wide to accommodate the water tank and the bulb is edgy, unlike what you see on other cruising boats nowadays. The water tank on the A55 is not in the keel, so the keel could be designed like a modern performance keel. It is much narrower at the base and the bulb is shaped much rounder. As a result, the A55 sails well upwind, very well for a ketch that is this heavy. It makes it practical to sail upwind in 6 - 8 knots of true wind, pointing 28 - 30 degrees to the apparent wind. This means the boat will tack through 95 degrees with only a modest amount of leeway, much less I bet than the prior models with their wide keels. As the breeze picks up to something like 14 knots, you will be able to sail closer to the apparent wind, 26-28 degrees, tacking through 90 degrees. As a result, even in light air, you are able to cover 30 or more miles upwind during a normal Mediterranean day to the next anchorage without using the engine all the time. I enjoy that sort of thing and I bet I would be much less happy with one of the prior Amel models.
A couple more things which don't go to your question directly but were very important to my decision to spring for a new A55 over a used A54. First, my wife much preferred the light colored wood inside and the big hull windows of my A55 over the more traditional look of the prior boats. Second, we liked the 3 cabin layout with 3 fairly evenly sized cabins, which enables us to sail with friends and family in much more comfort and privacy than in the prior designs. We have sailed with 6 people for many weeks and everybody has a great, fixed bunk and ample personal storage.
Good luck with your choice!