The advantage of both the Boeing offerings (767/777) and Airbus (A330) is the two engines. The outer wing can take pods clear of the jet wash. Anyone done any trials astern the C-17 wing?
Fitting a boom means a ramp swop each time, is that possible or cost effective? Ans what's the fatigue cost? We had C-130 ramps fitted for a HDU for Falkland Island support. The FI usage killed the airframes.
Since the C-17 would be only able to do either freight or AAR at once, with a long re-role turn round time. So I think the flexibility would tend to turn out to be spurious. And with the higher running costs and the expense of having to hire AT to take the troops when the C-17 was toting the fuel, I can't see there being any cost savings.
If there were, I'm sure Boeing would have pointed it out......