I have no insights into what Limesurvey GmbH can offer and what not. I am only providing my view as a user of Limesurvey and my experience in the past.
Any details regarding this must be discussed with Limesurvey GmbH directly. But as Carsten has already respondet in this thread, he might see your questions and respond here, without the need to get in touch with them via support ticket or similar. Time will tell.
Sure, if we subscribe to a comfort update key in a way to sponsor that?
But I highly doubt that a comfort update subscription will be enough to "sponsor" a more indepth testing for PostGre. I think that is a totally different beast. The comfort update subscriptions as a whole sure help to support Limesurvey and its development. But I highly doubt that one additional CU subscription will pay for the additional resources to do thorough testing for PostGre support. I mean, PostGre is supported and generally works. But while I am sure that there is some basic testing by the core developers, at the end the real life test (like with many other softwares today) is by the end user. So it also depends a little bit on the community to test during alpha and beta stages as well as reporting issues in the bug tracker. Reported issues usually get resolved fairly quickly in Limesurvey, in my experience. Just like these issues seem to be resolved in then newest release, according to Denis.