I'm pretty sure the KC-46A uses a standard 767-200ER wing (earlier RFPs were based on the 767-300F and perhaps even 767-400ER wing).
Boeing would have done wind tunnel and CFD testing prior to first flight and felt they had a handle on it based on both, but that is why certification agencies require the OEMs to actually fly the plane because modeling is not always 100% accurate.
Didn't Boeing have to strengthen the 767-300ER wing for the KC-46A? No civilian versions of the 767 ever needed to hang anything other than one engine off each wing. The A330 MRTT was able to use it's wing's heritage as being designed for both the A330 and A340 to its advantage. The A330 tanker attaches the WARP's to a hard point that is located where the #1 and #4 engines would attach on an A340. I don't see how a modified 767-300ER wing could be considered a standard 767-200ER or 767-300ER wing.