Although Shimano state 28t Max for a short cage rear mech it will operate fine with a 30t but no bigger.
The short cage will probably not wrap the (OP assumed) 50-34 (compact) plus an 11-34 properly (16+23). 39 is way out of spec. This would not stop you using that configuration but be aware that in the event the rider strays into small to smallish, the cage will have run out of 'spring'/rotation and the return chain will foul the chain on the tension jockey wheel (noisily - so the rider can do something about it - shift to the large chainring). Or the rider deliberately runs a short chain, resolves never to go large to largish, and realises that if they do, damage will likely result (to chain or RD or both).
The hanger extender to which
@Cycleops pointed just drops the guide jockey wheel down so it doesn't 'interact' with the (over) large sprocket. Its use has no effect on the 'wrap capacity' of a short cage RD.
From
Sheldon "Downgearing Your Bike":
"With the chain sized as above, this Ultegra short cage derailer will wrap enough chain to run in the 3rd from the high gear (an 18) and the small ring (30). This means that this short cage derailer will wrap ( (32+52) - (18+30) = 36 teeth maximum. Shimano's specification for this derailer is 29 teeth total with a maximum 27 tooth cassette cog. If one pushes the derailer to it's limits, it is possible to exceed Shimano's recommendation by a few teeth, but not a lot."
"Shimano specs its equipment for neophyte riders, who may push levers until something happens. They can get into, and ride in, the small-small or large-large combinations without being aware that those are not good combinations. Therefore, for experienced riders (who don't ride in the 'small-small' gear combination), it is certainly possible significantly to exceed Shimano's recommendations and get by with a short- or medium- cage derailer shifting a 'mountain' cassette."
