interesting observation about the long cage induced "slight delay" (that kind of sums it up doesnt it?) and good to hear that it works well for your 40 pound lump. I guess because I am so used to triples, I could never see myself shifting into large-large (touch wood eh?), so I'm surprised you changed from the med. I guess though that with a modern double, the cross chaining is so much reduced angle wise, so it probably is a good thing to cover the bases for not ever having an issue with the rd being overworked in that regard.
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I don't like to cross chain, I just don't like having any combos that stress the chain even if I avoid them, but I'll give the medium cage a try on a long ride next week to compare. I suppose shifting efficiency isn't ever going to be as slick as with the previous 12-25 and short cage Rival derailleur because there are is so much more chain to take up between shifts.