Luckybreak,
Don't think we didn't think of refitting the twin 20 mm. Just look at the 303 thread <g>.
Unfortunately, while we had the endurance we did not have the range. One of the planning options was for us to fly from ASI to the FI and remain on task . . . PLE was not an option.
We calculated that this would afford the fleet 2 hrs 30 min AEW time until the 5th aircraft had to ditch.