Originally Posted by cyccommute
As with most things bicycle, I'd say try the long cage with a 32 or 34 tooth cog first. You might be suprised. I've run 30 tooth cogs on a short cage Ultegra before without ill effect. I'd suspect that a long cage would be able to handle the chain wrap quite well. Put on the cassette and put it on a work stand first. If it shifts funky, start looking for a new rear derailer. If it shifts nice and smooth, save your money.
I agree to a point. If I already owned it, I'd give it a try to see for myself. My bet, however, is that it isn't going to work.
The issue isn't chain wrap, it's that the upper pulley rubs against the biggest rear cog. You can dial in the B-screw to pull the derailleur back, but you'll probably run out of travel. One possibility is to find a longer screw. Another is to remove your B-screw and install it from the other side so that the head rubs against the dropout. That'll give you a few more millimeters which may be enough.