A number of us are quite fond of our Hy6 systems, with its quality lenses and flexible design. Many in the US use Leaf backs, as the Sinar backs were not well marketed here (if at all). The issue with Sinar backs was that you were locked into their processing software, but I think the esprit series was able to store DNG files directly as a workaround (but never seen firsthand, so you may want to check this...). People who have used the Sinar backs have commented on the quality of the files and the color.
In general, the Hy6 was the same camera regardless of the name on the top (Leaf, Sinar, Rolleiflex). But check with someone like Eric Hiss (Rolleiflex USA) if there were any particular differences (such as battery setup) on a Sinar body. Not sure if Sinar still services them, or are you in aftermarket.
Hope this helps.
