I have proved the point to myself on how to break in an ABC type taper bore engine. I bought two OS 32SX engines brand new and mounted them to two run stands with 100% identical set ups. I even took them apart to insure there was no remaining machining dust in them. I went so far as to torque all the screws on the engines.
Engine #1 was broken in and run slobbery rich in a cold 4-cycle mode
Engine #2 was broken in slightly rich from lean

Identical amounts of time (not fuel, since engine #1 was consuming it at a greater rate per minute than engine #2), I put a total of 20 minutes of run time on each engine in 2 minute intervals with complete cool downs between runs.

After the 20 minutes of running in, I started leaning into them and would take readings at the absolute maximum rpm. The engine broken in with a 4 cycle richness, was roughly 500 rpms weaker than the second engine. engine #1 also seemed to have a bit loser feel to it although I did'nt measure where the pinch began relative to the other engine, but it was loser

Is 500 rpms a lot, in my opinion it is, especially when I intended to use the engines on a twin. I installed the engines on a couple sport airplanes and tried to fly both of them similar amounts. What I noticed was that after about maybe two or three gallons of flying time through the previous test engines, engine #1 was significantly weaker than engine #2.

These are just my experiences a few years ago. I have been a student of the George Aldrich and Dave Geirke break-in methods for taper bore engines, so don't flame me- I believe they were 100% correct.