what you say is generally true, Razorback.
even the biggest/key star QB's like Ben and Brady shouldn't actually move the line to that extent. but as you know it's about dropoff with the replacement, style of play, etc. once people saw that JimmyG was good, the Pats were still favored (just slightly less than they'd be if Brady was playing).
obviously, Ben to Landry Jones is a bigger drop than say Bridgewater to Bradford. also, the Steelers sling it around and Roethlisberger is an elite QB/leader and Pitt is a very public team so Vegas will of course make a considerable adjustment (perhaps too much, we'll see) cuz they know everyone will be on NE.
all that said, if you can hold your nose and bet on Landry Jones AND against Belichick/Brady, there's a lot of value in Pitt +7.5.
this could be a kitchen sink game where everyone rises to the occasion, not just to elevate their play to make up for Ben's loss, but also being home vs. the Pats (and off a bad loss) they should be focused.