Interestingly, the "official" specification for the GS derailleur is still only a max. cog size of 28 teeth, per:
http://techdocs.shimano.com/media/te...9830702204.PDF
It's ability to absorb additional link capacity was engineered around the use of road triples, not neccassarilly wide rear ratios.
However, as you and others have experienced, it's capable of handling more.
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