Actually, I'm going to offer a contrary opinion. I've never tried it with Ultegra, but I had a Tiagra road derailleur setup once with an 11-32 cassette. It shifted OK, but the derailleur hopped a little when the chain was on the big ring, like it was bouncing on shift ramps or something. I swapped in a MTB derailleur because that was my primary bike, but it would have been mostly OK like that.
Of course, this is assuming that you have the mid-cage derailleur. If your derailleur is short cage, it won't even be close.