Originally Posted by
daoswald
Kind of doubt a 30t will work with 5600 series GS derailleur. 28t was even cutting it close without a hanger extender.
I expect tightening the B-screw a bit would have solved that problem. I have a Shimano RD-6503 (9-speed) rear derailleur also rated for a 27T maximum on one bike and it appears that snugging up the B-screw would easily allow a 28 or 30T large cog.