how long is a piece of string?
Or in this case a short cage derailleur.
What size parallelogram on this derailleur?
Without information on the derailleur it is hard to say.
In the big front and big back combination doss there appear to be some extra slack that can be taken up?
Is the chain long enough?
So many questions so few answers.
It is not a big job to take the cluster off to give it a test if you have access to a 27 or 28 tooth cluster.
Looks like its all systems go for the 12-27!
Great question and great responses. I'm thinking of changing to a similar setup..from 25 to 27 or 28 on the back. I've done about 4500-5000km on my current setup and the general consensus from a question I raised a couple of weeks ago was that it'd be best to match a new cassette with a new chain. So I'll be thinking about it once I've done a few more k's.