Originally Posted by the beef
Why wouldn't it be possible? It should be fine with the proper adjustments, which seem to have been made. If you don't seem to have problems, then you're correct - it's just fine.
Uh, because the Ultegra rear derailler is rated as being able to handle a max cog size of 27t. That's why he's asking - he's surprised that it works.
And I'm surprised that it works, too. That said, if the derailler cage and/or pully wheels aren't rubbing against the cassette, then I think you're good and in business. If this problem does occur, then you could just slap a LX or XT rear derailler on there and be in good shape.
I'd be interested to see some pictures, if you get the chance to post any. Pictures with the chain in the 34t cog, taken from an angle where I can see the rear derailler's top pully wheel.