Maybe I missed in your post...what fuel???
If unleaded pump fuel,it will not color the plugs very well and bone white is good.
I didn`t see where you posted any E.T. or MPH change with jetting up?
I`m sure you know that if you can`t slow the car by jetting up you are out running fuel supply.
Maybe borrow a carb, to rule out HSB fuel curve variables.

And..... what are you running for a scoop /fresh air arrangement?
I made a Ram-Air set up on my streetcar..... REALLY disturbed the fuel-curve on my wideband......
Oh,and one more thing......2/3 mph from March to summer humidity/barometer/heat is not out of the norm.
Any other cars you know that have been there at both dates you could compare notes with????
I`m never happy going slower.............. but March to July is a BIG variable....