Completely forgot about that when replying. You can still get a good reading after 150 miles, but there is so much more to reading a spark plug than taking them out and looking at them closely, it is a deep skill that I don't have and unfortunetly one that is lost through time. Nothing better than a wideband o2 reading.
Good discussion... I enjoy getting into the weeds a little.
I am long ways from any expert spark plug reader... but...
I don't believe that the spark plugs have ANY clue when they fire the mixture inside a combustion chamber whether it comes from an injector or from the venturi and jets from a carb. The spark plugs also have no clue how many miles on there is on the engine. So reading a spark plug from and FI engine or a carb'd engine will most likely show the same "reading" analysis. For the past 40+ years I've owned, maintained, and tuned both carb'd engines and FI engines, and the spark plugs can be reasonably accurately read based on basic visual color and plug electrodes and ceramic analysis.
Yes, a wide-band A/F gauge would be a great diagnostic and analysis tool. I believe reading spark plugs today is probably more of lost art than it was 40 or 50 years ago. I don't read many posts where a guy actually pulls his plugs to "see" what's going on with the mixture. However, I don't think reading a spark plug to determine the results of basic combustion conditions going on inside of the chamber is that difficult.
In my "non-expert" opinion, the plugs looked like they were seeing a lean mixture and perhaps running a little on the hot side.
But with that said, I will happily defer to any expert opinion out there.