Hi Vito,
as far as i recall Thorstens arguments, the last time he was asked, he said, that there is no real benefit in moving up to a more modern core, as the SIDs can only accept a moderate register update speed - so basically, all the bases are covered with the PICs... development would be a major effort and there may be other low-hanging-fruit goals (aka other projects) in the limited devtime available... but this is all hearsay :-). The great thing about MIDIbox is, that everybody can step in and develop to his/her hearts desire, it is all documented, you could give it a try!
Many greets,
Peter