I am with Sue on this one. These sorts of things are usually dose driven. Plus, your doctor has to give you all those worst case scenarios. This is not to scare the crap out of you, but to protect his hiney, just in case there are complications.
Another point to consider is: what are your options anyway? You have only three choices. One is to go forward as planned and hope for the best. Two is to stop your Imuran before surgery and risk relapse. Three is to refuse surgery and continue with chronic sinus problems. Seems to me that option one is really the only good option.
If it makes you feel any better, I had an adrenalectomy while taking Humira. My surgeon and the anesthesiologist really had no opinion regarding whether it was a factor or not. So, just for my own comfort level, I skipped one dose before surgery. Surgery was fine (and there were loads of possible bad things that can happen with an adrenalectomy), and I did not relapse. So, all was good.