Some years ago now I was involved in a research project comparing tanker capabilities. Rather than relying on manufacturers' glossy brochure claims, we set the ground rules for comparison. All aerodromes would be sea level with 10000ft RW length, still air and ISA. The aim was to establish the max offload available in a set time period on an AARA at a given distance from the departure aerodrome, with recovery to the same aerodrome to land with the equivalent of an hour's transit fuel burn (to simplify diversion assumptions).
All participants were requested to provide the actual RTOM for the specified conditions and the max offload within the set criteria.
The US participants immediately requested a 12000ft RW; this we refused.
When the results came in, the KC-46 was unable to take off at anything close to its MTOM; the A330MRTT was the clear winner and even an A310MRTT with 5 rather than 4 ACTs came very close to the KC-46.
Back when ex-ba 767s were being considered as potential RAF Future Strategic Tanker Aircraft, the Boeing rep admitted "That's where Airbus has us beat" when I queried the figures shown in Boeing's own brochure, which indicated that the 767 (admittedly not fitted with the uprated engines/brakes of the KC-46) would struggle to be able to take-off at MTOM from Brize Norton except with a significant head wind and low ISA deviation. Mates who flew the thing in airline service confirmed that it was a real slug departing Africa with a full load.