Ford might not want to make the info available, but Hella has published this:
http://www.hella.com/startersalterna...rge_system.pdf
I suspect the following comment in the Century Yuasa info sheet is a major reason why the OE Ford batteries are silver calcium, and the replacement Motorcraft batteries are not:
This provides car exporters with an increased chance of having the batteries retain sufficient charge to start the vehicle by the time they reach their export markets.
I really cannot see any specific characteristic of silver calcium batteries that makes them necessary for the correct functioning of the Ford Smart Charge System. The main issue is the over voltage that can occur, but silver calcium is not the only battery technology that can cope with this.
Anyway, Ford Europe now supply Motorcraft calcium-calcium batteries as an alternative to the OE Ford units, so they are obviously quite happy for this type of replacement battery to be fitted. I'm not sure who makes the European Motorcraft batteries.
Finally, the fuel consumption at idle sometimes drops since I fitted the Century battery, which may indicate that the alternator is not charging. I hadn't noticed this to the same extent with the OE battery. It seems to happen most often after a long run, not so much during city driving.
I'd say just buy whichever aftermarket battery you like. Might be worth avoiding the cheap Made In China stuff, but that applies to many things!