I think I know part of the answer but after some advice.

The wife has ordered a new cx bike it comes with a Shimano Ultegra CS-6800 11-spd cassette, does this mean her old wheels (deore XT FH-M756-A) cannot be used with the 11 speed cassette, look like yes?

if this is the case, is there any cheap solution such as using an sram 11 speed cassette to use the old wheels, looks like no ?

Thridly and the reason of all this messing about Iron cross rims for the three peaks – good or bad idea? All the blogs seem to suggest high pressures are the way forwards but iron cross seem to be a bit focussed on low pressures, the original plan was to use either an old mavic rim or an aileron with landcruisers pumped up hard but may not be an option with the rear wheel now:cry:
Another alternative if the iron cross may not be up to it is get a Mavic Aksium One rear wheel and stick the casette and landcruiser.

Any thoughts gratefully received.

in answer tot he first two questions, correct on both counts.

No idea on the third, but do let me know as i am looking for exactly the same thing!

Iron Cross are 45 psi max I think for tubeless – if using tubes you could use more pressure.

bump for the evening crowd

As above on the wheels, it’s only low pressure for tubeless. You can use higher pressure with tubes in (I use 70psi on the road)