I dont really have an opinion on exhaust because I've never really messed with it besides dropping empty cans into a friend of mine's diesel stacks.
I did find that a flowmaster 40 would only add .1-.07 seconds to my 1/4 mile time (even that is questionable when I compared the 60' and 1/8 mile times). So all those heated arguments over theories and physics and them being restrictive are pretty much a load of bull IMO.
Bill does advise custom tunes for certain headers, high flow cats and other mods.
In regards to a lean condition, if it is done right you shouldn't have any trouble...that is my opinion, so me and your mechanic agree on that part.