Reporting back, might need to go for the 54 or look elsewhere...
Just got a reply re: seatpost length.
"44/49/52 use 300mm seatpost
54/56/58 use 380mm seatpost"
"52cm cannot use 380mm, the frame will be damaged"
"All seatposts must be inserted at least 100mm"
My current bike has a saddle height of 698mm from the BB, and from BB to saddle rails of approx 670mm. The Ultimate has a seat tube length of 494mm, so that's approx 176mm seat tube exposed. This is also with 172.5mm cranks, which I wanted to change to 165mm.
The X68 in 52 has a 456mm seat tube. To get the same saddle height for same leg extension, I'd need the same BB to saddle dimensions - so approx 214mm seat tube exposed (670 - 456). With the change to 165mm cranks, this would increase to 221.5mm. With minimum insertion of 100mm, I'd need the 380mm seat tube, which apparently will damage the frame (hard to see how, given the 54 seat tube length is only 17mm more - maybe the seat tubes are slightly different dimension?)