All color transforms currently restrict output to 8 bits/channel because this is hard coded. This is probably acceptable for monitor output although with display port we are starting to see monitors that support higher bit depths (10 and 12 bits/channel currently but display port allows for up to 16 bits/channel).
The CUPS pdftoraster filter will need the ability to set the bit depth to either 8 or 16 bits/channel but a more general solution would allow the calling app to set any output format that is supported by the CMS including floating point formats. So this will likely require adding the ability to set this to the poppler API.
*** Bug 15177 has been marked as a duplicate of this bug. ***