Stock EFI is limited to stoich, but carbs and "others" could go leaner.
I'm wondering if the hotter spark is most notable in "lean burn".
Correct, and a big part of economy tuning and reducing pumping losses. High-energy ignition (HEI, Ford TFI, DS-I, P-II, etc) consistently allows better fuel economy with wider plug gaps when tuned properly, than CDI (MSD, P-III, etc), and one reason the OEMs use it and not CDI. I and others have repeatedly shown HEI to properly light 0.5 to 1 AFR leaner than CDI consistently. CDI is great for rich AFRs, drilling through polluted mixtures such as radical cams at idle, 2-stroke oily fuels, extreme rpm, etc, but the spark is sharp and thin and lights fewer fuel molecules on each spark. One reason they fire multiple sparks when possible. That's the general gist of it and why CDI is usually not the top pick for street and most racing these days.
However, using HEI will not gain you economy unless tuned to take advantage of it. Stock EFI is limited in the AFRs it can run, and inconsistent without WBO2. Other factors such as constant high-throttle heavy load towing may not allow enough power at cruise for leaner tuning. So while HEI
should make better mileage, and is used in racing for this reason (less fuel consumption = reduced race weight or fewer stops), certain conditions or EFI systems may preclude effective economy tuning for certain conditions.
[EDIT]
63 Sprint said:
Cruising the AFR reads 13.7 - 14.2
Don’t want to go any leaner. No anti knock protection.
That's very rich for cruise, including stock. Typically with our engine designs, the low to mid-16:1 range burns cooler and much better economy. AFR heads can do this. However, the ignition
must be tuned for the AFRs to burn properly and not too late, causing latent heat and potential detonation. Ignition tuning is a must.