Whoever told you to use shims on an 850 is full of it as that's simply false as they don't exist for 850s. The older rear pads originally had shims but they've been deleted in more recent years too.
Volvo sells their very own silicon brake paste used since the 700 days (our dealer stocks it) and it's still the best stuff for all OEM pads to keep them quiet. I'd advise OEM parts 100% to prevent noise, warpage and for maximum longevity overall.....least problematic from from I've seen to date.