hello,
i have replaced a lot of ac evaperators on 850's and yes its a big job that requires dash removal. but only ever for a leaking evaperator that causes the gas to leak out of the system,there isnt anything else on the evap that can be faulty unless its cooling fins where somehow blocked by debris,which is very unlikely. i would seek a second opinion,if the ac works fine for a period then stops its more likely a electrical problem,if it works poorly then stops more likely a gas or pressure problem,either way i dont see how the evap can be at fault.hope this is of some help.

steve.