Finally i purchased an iPhone 5 without selling my iPhone 4S in order to compare both headphone outs with my music (aac 320) and my iems (UM3x and SE420).
To cut a long story short:
Last night my wife put me thru a blind test listening to the (fabulous sounding) album Aja by Steely Dan (MFSL Edition)
The outcome was embarrassing yet revealing: 4 misses out of 5 tries. I could no tell apart both iPhones.
Hence in my view paired with balanced armature iems with a mild impedance distribution the 5 sounds so damn close to the 4S that sound quality is non issue.
It's important to note that on the 5 you need almost two more "points" of volume to achieve the same sound pressure level that the 4S reaches with two less steps. I'd say that the iPhone 5 is powerful and the 4S very powerful.
Now i must sell one of the iPhones but i won't make the decision based on the sound quality.
The 5 is very good to these ears. The advantages of the 4S (lower Z, more output power) might show with wild impedance iems like (TF10's, SE535s) or with dynamic headphones but it doesn't seem to help to neither my UM3x or SE420. And if it helps them i just can not hear it.