I was just at a bike event last night with a bike shop vendor there with a mechanic. He said I wouldn't need a bigger rear derailleur than my OEM one to handle going from a 11T-30T to a 11T- 34T rear cogset. He said he only would have to add a few links to the chain for the bigger cogset too. The other bike mechanic said I would need a larger MTB derailleur. Can anybody tell me who is right?

Lars