Originally Posted by T-Mar

It's not necessarily that the derailleurs are rated conservatively but that there is quite a bit a variation in dropout hanger geometry (specifically length) and that there are ways to cheat. The longer the hanger, the bigger the cog size that your derailleur can accommodate. Pushing the wheel farther back in the dropout will also allow larger cogs but at the expense of a larger chain gap and shifting crispness, particularly with indexed systems. A lot of modern manufacturer use vertical dropouts specifically to avoid shifting issues caused by non-optimum position of the axle. Maxing out the B-screw also helps, but depending on the angle of the hangers stop tab, it may not be enough and require a longer screw. The bottom line is that there are enough variables that it is impossible to predict. You'll just have to try. There are some combinations where a 34T would work and others where it wouldn't. You could probably even find a combination where it wouldn't work with the spec 32T.