What if one cylinder was completely dead? It wouldn't show a difference during the magneto check. Can you check EGT individually at each cylinder?
In light aircraft with constant-speed props, you can get rated RPM and manifold pressure, even if the engine is weak. The blade angle will adjust to make the parameters fit. But it doesn't mean you are getting the expected thrust.
In a turboprop, you check torque. Not a 100% guarantee, but pretty certain you are getting the thrust you expect. (The exception I can think of is a bad blade). Obviously, not available in a C-182RG.
Double the takeoff roll? Even winter to summer shouldn't cause that much difference. I say have a different mechanic look at it before someone dies. Unless they did a lot of structural repair that significantly increased the weight of the airplane, I think something is wrong.
In my OV-10 Bronco Squadron, we had an airplane that took three times the normal roll to get airborne, and had half the climb rate of all the other airplanes. If it was in a 4-ship when we went to the bomb range, we would have to take extra passes around the pattern so he could get up to pattern altitude.
They sent it back to Rockwell twice and weighed it and checked it for straightness. They swapped the engines, and checked the torque on the ones they took out.
They never found anything. When the Air Force Museum said they wanted an airplane, it was the first one to go.
(If you've seen it at Wright-Patt, I've flown it, and it is this clunker!!)