GT-i9100 user here. (No bloody AT&T or Vodaphone versions, Exynos not OMAP).
Got myself a GC110. Pleasantly surprised to find the original battery for my old Galaxy S2 *fits the camera*. Well, not completely perfectly - I can stuff it in and it barely fits, and I'll need a bit of work to get it out, but the camera works, no power or voltage problems. I had it running up to 20% left too.
I first suspected as much when I realized the only thing different on the new batteries were the serial numbers. But... the snug fit I got here precludes the "taller" official extended from fitting, let along things like the huge Momax one.
Got an extra GC110 battery and charger as a bundle, the Galaxy S2's battery works on the charger. The GC110 extra battery does indeed work on the S2, and by extension the GC110's original battery. So now I have 3 batteries and 2 devices. Joy.