Does anyone have a view on how much the use of the air conditioning system affects power/fuel consumption on the Yeti?

No, but switching in and out of "econ" mode on Climatronic makes no measurable different to the instantaneous mpg mode on an Octy once you've achieved cabin temp (maybe 2 miles from a start unless it's very cold out).

The air con on my old Octy 2 1.9 DSG used to cost me about 2 miles in the gallon.....

I've had aftermarket aircon fitted to two cars some time after buying them, and in neither case did it make any difference whatsoever to fuel consumption. There may be a difference with underpowered cars - or in traffic - or if you drive around in the height of summer with all the windows tightly shut. But I don't think it is worth worrying about.

It depends partly on your engine size. The effect on a 2.0Tdi should be pretty negligible and not worth worrying about. A petrol 1.2 may have around a 2 mpg impact in the height of summer when it has to work hardest.

The question is "is it worth 2mpg for the comfort of aircon"? For me the answer is yes but each to their own.