Anyone us a Dura Ace 7700 GS Rear Derailleur with a XTR 12-34 cassette and a 34/50 Crank? The setup is within the maximum capacity - but the specification says max sprocket is 27 teeth.

The reason that derailleurs have two specifications (capacity and max cog size) is that neither one is sufficient to define the derailleur's performance. Most rear derailleurs will handle a cog that is a couple of teeth larger than specification, sometimes even 3 or 4 teeth, but NEVER 7 teeth. To get the fast and precise shifting of a racing derailleur, the upper jockey wheel is positioned close to the cassette cogs, and the derailleur follows a path that keeps it close to the cogs throughout the specification range. An MTB derailleur tracks a different angle, in expectation of a much wider range of cogs. The DA (or any road-specific unit) would probably hit the larger cogs on your 12-34 cassette.

Kerry is correct in theory. But............

I have a 32/44 double on my MTB with a 12-34 on the back. I use a dura ace double rear derail.

Upper wheel tracks very well. Maintains a good steady gap. The wheel position seems mostly defined by chain length or chain tension.

And I am not the only one. COUNTLESS pro downhill racers I know use DA rear deraileurs with 12-34 and 12-32

I used an Ultegra GS (for triples) and a 12-32 cassette on my MTB with with no problems. My chainrings are 48-36-26. With a minimum of chain on wrapped around the 48x32, there is just enough clearance on the RD when I go to 26x12. Going with a 50-36, I don't see a problem. If you do you'd just have to rule out using the extreme combo's. Good luck.

Oh BTW, the RD tracked very closely to the 12-32, but worked well. I don't know how well a 12-34 would fit. It's not much larger, but there wasn't much space.

I don't have it set up anymore on my MTB, otherwise I would check it out. I eventually went with an XTR GS (shorter than the SGS version, but about the same as the Ultegra GS).

Maybe it depends on the hanger length? I got one bike that will barely do a 27 cog without it hitting the pulley. Most bikes will do a 28 cog, but that is about it and 34 is simply dreaming.

Thanks to everyone for the information. I am going to try out some combinations.