If it were me, I'd go with an XT (771 or 772) RD and an XT 11-34 cassette. A 12-34 would be better, but I don't think anyone makes one light enough.

Nothing wrong with running lower gearing. I've gone to a triple with a 32t and may go with a 34t one day. Truth is, if you can't push a 34-28 up hills, you probably won't be pushing a 50-11 very often.

John