....
EDIT: There is one place where groupset affects shift quality, and that's in the minds of LBS mechanics. Until I learned how to do my own work, I took my bikes to the LBS. I was actually told (by tow shops), "It's Sora. It's supposed to shift like crap." And when presented with the same grievances on another bike, teh same mechanics told me "Yeah, 105's supposed to shift better than that" and they got right on it.
... .
+1
And that can even differ from mechanic to mechanic within a bike shop. Some take a bias against anything older or cheaper and instead take an elitist attitude and scorn anything that isn't up on the race/high performance level.