I put a 9sp 11-30 cassette on my wife's bike with an Ultegra RD and it works great (a 32T cog was too much) so the 105 should handle it, too.
Before you ask, I don't know where to buy an 11-30 9sp cassette; I made mine from a standard 12-27 Ulterga by dumping the 14T cog and adding a single 30T from an 8sp LX. Just make sure to use the 9sp spacer, NOT an 8sp one.
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