I believe the idea of using the 604's is for endurance/ range/ speed...
after MH370 went missing and biz jets where charted for the search as the 328 lacks range- it was obviously realized the dash 8 simple doesn't have the range and speed for a search/ rescue far out to sea....
from what I heard the Orion's are being replaced with specially built global hawks in the distant future... probably within the 20year contract term...