is there any real/noticeable difference, looking at running a shimano Ice tech in an Avid Elixr.

que Avids are crap just buy Shimano jibes

Shouldn’t be a problem as far as I am aware.

I think all companies use the same thickness rotor (1.6mm???) with the exception of some Hayes brakes which use a thicker rotor (1.84mm???)

Someone will probably correct me on this though

Shimano RT76s are ime thinner than Avid G3CSs and Superstar floaters.

They’re fine with Shimano calipers where the servo-wave takes care of the increased pad/rotor clearance but not so hot with Juicys/Elixirs there is too much lever movement at the start.

By the same token Shimano calipers with Avid rotors have virtually no modulation.

Surely the calipers in an open system will adjust to take account of any variations in pad / rotor thickness?
The only issue I can possibly foresee is using a rotor thats thicker than a brand new manufacturers rotor with brand new pads which may not fit with the pistons all the way out.