The point just prior to the moment your city begins to run intolerably slow. That point will be both arbitrary and different for each person/computer. And that's the whole point. The maximum city size should have been a configurable option from the start.
I tried the mod and ran across an odd problem. While it moved the border markers out, the editable area remained the same. Thus, I simply had a frame of land all around my city in which I could not place anything. I was wondering if it was just me. Apparently, the mod is not quite ready for prime time just yet. I look forward to future Orion Project updates.
I recommend everyone try it. If it works for you, great. If not, then restoring your game is as simple as renaming or deleting the file. Couldn't be more simple.