The speculation is that any Easyjet move to T2 will use the ground floor check in desks. Easyjet at 6 or 7 aircraft, IMO, would be able to use T2 in it's current form. Assuming KL move, next Summer in T2 there will be no KL or OK nightstoppers plus most Easyjet flights will have departed or be departing by the time most of the Transatlantics arrive. Any other U2 arrivals/departures will be few and far between until when Lunchtime/afternoon when there is likely to be 3 or 4 aircraft down at once or close to each other ay which point DL, VS, SQ, US etc have departed or ready to do so.
Anything above 7 will definitely require the said expansion but that goes for all three terminals.
As I say, Easyjet aren't unhappy with T3 and I even get the impression that they want to stay there but they are concerned with the space at the terminal and that is why it is still a possibility.