Indie,
You are right in that KC-135/KC-10 boom system can transfer fuel at about 6000lbs/min which is about double that of probe + drogue. However, the RAF P+D crews were far more flexible in their operations which allowed (generally) a muck quicker join up over USAF and ANG crews.
Put together and you are on a winner. I never understood why we didn't order a boom system as well (cost obviously) as we operate with so many coalition partners who only have boom systems.
Regards