Lots to respond to here.
First, you make it a point to claim that Webb is good and will be good at RT. So...why would the comparison be between Bushrod and Webb if Webb is going to continue to be a starter? Wouldn't the sum total be Bushrod replacing Carimi/Scott in the starting lineup, allowing Webb to move to the less pass-block strenuous RT? How could the Bushrod signing be viewed as anything but a positive for the Bears?
The point of this thread is to compare Bushrod's body of work with Webb's body of work and discuss the potential fact that the Bears likely overpaid to sign a very mediocre left tackle. The Bears didn't have a lot of free agent money to go around so it's a simple discussion of whether the money could have been spent elsewhere.
Second, the Bears didn't invest THAT much money in Bushrod. You always overpay in FA, but the Bears/Emery were kind of stuck in a corner because they had very little OL talent. Lovie was fired because of his bad OL. I don't know what you are suggesting as an alternative to the Bushrod signing...should the Bears have stood pat with Webb at LT?
The Bears could have stood pat with Webb at LT absolutely. Carimi had a knee problem and was a tremendous run blocker. He was a tremendous run blocker because that's what they did at Wisconsin. Carimi was learning to pass block and his knee was in need of rehab, he didn't get to rehab last off-season because he had three surgeries. If Carimi's knee winds up healthy and he becomes a better pass blocker the Bears could have had two young cheap OT options.
Third, one of the key components is how much help Webb needed, and how that limited the Bears to having only 2-3 pass targets on pass plays. You always gloss over this aspect, making some subjective claim that Bushrod received as much help as Webb (highly dubious), yet this was the main reason why the Bears needed to move Webb from LT and sign Bushrod. How many plays did Bushrod receive help? How many plays did Webb receive help? If you can't answer that question, then this discussion is kind of pointless.
Your assumption is that Bushrod gave up more pressures because more often he was out on an island...but you have no evidence to back up your assertion. This question isn't even worth answering until you provide evidence that Bushrod received less help than Webb did. You didn't watch the Saints play last year and you've likely not gone back and watched the coach's tape.
Fourth, the whole "pressures" stat is not very objective. In most quick passing spread offenses, the OTs are taught to not let anyone beat them inside. If the DE has to go outside, by the time he runs around you the QB will have released the ball and the DE will have taken himself out of the passing lane. The Bears OL in general and Webb in particular were TERRIBLE at allowing inside pressure...quite honestly the worst I have ever seen in NFL history. So perhaps these "pressures" that Bushrod gave up were partly by design.