Hi Phredd,
The CD5 by JDS Labs costs £250 and is clearly a top class high end hi-fi product but whilst doing my research I couldn't find anywhere where I could turn up with my phone and headphones to plug in and have a test drive. Also, no-one was able to give me a clear yes or no as to whether they could use the lightning port to access the music signal in order to bypass the phone's DAC. It would be great to find this out once and for all.
The C5 costs £189 and doesn't have its own DAC but like the CD5 is a box which is bigger than the phone. Also, neither of these undoubtedly sonically wonderful solutions allows use of the iPhone's inline contols. This means that taking phone calls becomes a faff, especially if you have gloves on. Phredd, you are totally correct that the Fiio Rocky at £23 is not the highest hi-fi going but it is small and good enough for the majority of users on the move who need the inline controls. I must also say that for £23 the Fiio is surprisingly well made.
You may also be correct that a headphones' impedance may not equate exactly to an inverse of a headphones' sensitivity but I think in general terms the most expensive headphones have high impedance and low sensitivity. It would be great if we the users could compile a list of high sensitivity headphones that really work well with the newer iPhones . . . I haven't found one yet but I'd love to be guided by the experience of other informed users.