I did a century using the 11/32 cassette with the Deore XT rear derailleur on Miss Lightweight and I'm really, really happy with it. While the century I did today did not have a lot of climbing, with the faster pace and rolling hills it was sufficient to run it through the gears, see how the front chain ring worked, etc.
I was really worried that the shifting would seem really sloppy going from a 10 speed DuraAce rear derailleur to a Deore XT RD with a mountain bike cassette. I was very pleasantly surprised how it shifted and spun under load. While it's certainly not as crisp, it is much more than adequate. In fact, I would put it on par with my experience with an Ultegra setup.
The century I did today was pretty flat so I'm not sure I even got in the 28 let alone the 32. I did use the 50/11 several times, especially going down some descents and also closing a gap a time or two. The spacing of gears works okay as well. There's a couple of gaps my preference would be to have something between a gear but that's just a matter of getting used to adjusting your cadence.
I do have a little chain rub on the front derailleur that I didn't have with the 10 speed chain but I can tweak the derailleur using the 2 positions of the front shifter and get rid of it. I suspect that's just the difference in the width of the 9 speed chain versus the 10 speed chain.
I have a really hilly mountain century in two weeks and now I have a lot of confidence that I can survive it with this new setup. Having the 28 with the 50/34 is going to be nice. Having a bailout with a 34/32 is going to be terrific!!
I hope to get up to the mountains for a ride this week to give it one more test with a lot of climbing. If anyone would like more details on the setup just PM me. I'll be happy to share what I've done.