Well, the A310MRTT could have had 5 x ACT, but when the numbers were crunched it was realised that an increase in allowable MTOW would have been needed if max fuel was carried - and the Net Flight Path limits at the tanker's MOB were significant even though the RW length was not. So 4 x ACT was the final choice. In the case of the A330 though, it already had a 111 tonne max fuel capability, so it was retained for the MRTT, although with the added structural mass it is rare that 111 tonne can actually be carried, I gather it's normally around 109.
KC-46 needed some centre tank extensions over the base -200ER, but the number crunchers came up with a compromise which met the spec. and still allowed some cargo capacity with full tanks - as was the case with the VC10C1K. Fair enough, but it could have carried more fuel had that been necessary.
With you on the Iron Maiden! Wretchedly horrible device - and if you missed the boomer would then move the thing, losing you the previous visual reference. After my first couple of jousts, I had to ask the young lady driving the boom to keep the darned thing still. Fortunately when in contact the onload rate was vastly superior to that from a Victor. Having completed my Teach Yourself BDA Prodding lesson, the Russians we were after decided to turn for home before we got much further North.