I posted my experience in the 41, but we all know what happens there. It probably would have been better to post it here. This is just a short note for those of use who need a little bit of extra help getting up those hills.
Briefly, the new Shimano Ultegra 12-30 CS-6700 cassette is available. My shop ordered two and I pick up one of them tomorrow. I had to ask, and it helped that one of his suppliers were in the shop at the time. Might pick up both of the ones he ordered.
Earlier this week, as a test, I fitted a Tiagra 12-30 on my Dura-Ace 7900 rig and it works just fine. I'm running a compact on the front and up until now, a 12-28 on the rear. I did not have to add any links to the chain and the short cage RD handles the extra two teeth okay. Note: Shimano says that this combination won't work, so I can't guarantee that it will work on your setup. I just know that it works on mine and that I will welcome that extra gearing when I hit those 9-11% grades. Mount Baldy Ski Lifts ... here I come.
Posted just for info. More details in my post in the 41 if you're interested.