Engine Break-In Fumbled... Worrysome?
 

Fully rebuilt my '81 911sc (new P&C's, cams, SSI's, the works).

So... Embarrassed to admit this...

Car had trouble idling after rebuild. After months of messing with AFR meters, idle mixture screw, gas analyzers, etc. etc. to fix idle issues. Finally gave up and started driving at increased idle (~1100, to prevent stalling off of acceleration). After a few trips, I noticed one side of muffler was getting hotter than the other. My first reaction was that one of the cylinders probably wasn't firing... Pulled all the spark plugs and, low and behold, one looked brand new...

Turns out the ignition wire wasn't seated correctly on that spark plug...

Put all the plugs back in and insured all ignition wires were installed correctly. Car runs peeerfectly now!!! AFR sits 'exactly' where it needs to be when warm - 14.7. Idle is smoother than ever. Car scoots better than ever! - I love it!

However, now I'm concerned about the piston rings not properly seating (particularly on the cylinder with the spark plug unplugged) ... I drove it ~70 miles like this... Should I be worried?

I have a compression tester coming in the mail. Was gonna put some few hundred miles on the car, then test all cylinders... But it's keeping me awake at night...

Change the oil to prevent cylinder wash down. You’ve got unburmed fuel in your oil now. Then after that I would just continue with the break in as normal.

I would do the compression check after a while, but I wouldn't be losing sleep over it. Worst case you're out one piston, one cylinder, one ring pack, and some gaskets. But I think that's unlikely.

Nothing to lose sleep over, whats done is done and can't be changed, it is what it is. Continue with the brake in (which I would do very hard, specially in this case).
An oil change might not be bad as Matt said.