Gents, thanks for your contributions - I think I got the answer I wanted. Bit a thorny topic me thinks. I tend to agree with Max Contingency, SAR has to move forward and the MIRG should adjust their plans accordingly. In other words, if it doesn't fit in an AW139, then don't take it.
With respect to the AW139, I think there are a few more important factors to consider. As ever there will be teething problems, and if I am a betting man there will need to be some serious modifications to traditional hoisting SOPs once the crews have got used to the platform. I remember the Candians having all sorts of problems with the position and fleet angle on their hoist when the Comorant was introduced. Sadly, this is one area that consistently fails to attract a rigorous operational test requirement before release to service. In any event, good luck with the both the AW139 and the S92.