Tammo,
you taught us something
There WAS an Ace Cat 550!
It IS a different hull
Look at this ad:
The giveaway is the "step" in the transom.
Also look towards the bottom of the add where they refer to the pending 6,55m (misprint?) which is the 555 I am sure,
and the pending 5,0m which is the 501 I am sure
I like your T-top...the lines blends in nicely with the boat
Carlos Moran on this site had/has a boat like this I am sure, and also swears by it
The pic is somewhere on the Ace Craft page