The usual great wealth of experience in the responses. I'm relieved that no one said something along the lines of "why bother, the xyz laser is only a year from deployment and it will fit in an ADEN gun bay".
I'm a self-confessed fan of cannon in aircraft, the more we are told about the good reasons for not needing them, the more we seem to need them.
The sad part is the fact that guns like the ADEN are still in service, and they are basically a German WWII design - where is the product evolution.
A direct performance comparison between the ADEN, DEFA, NR-30, Oerlikon in similar calibers would be fascinating (at least for me). Until then the consensus appears to be that the 27mm Mauser is probably the best single barrel high caliber multi-purpose cannon out there today, which is fine by me.