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Just playing around with the AE setting in 12P and see it drops to 10.5:1 when I snap the throttle. Is this what I should aim for or a bit leaner?

I personally dial it in so that the AFRs remain flat when jabbing the throttle, as long as there is no lean spike and stumble is should be fine but too rich it would be wasting fuel and puffing black smoke when you peg it for no good reason.

Thanks for that info. I really had no idea just how rich an engine will go when you stab the throttle but I can relate to a carbie where you see a squirt of fuel going in but I never looked at a gas analyser to determine if it changed the ratio. Looks like I'll trim it back.

I don't have an answer for that but what I have done on the dynojet dyno's is play with AE till it lost torque on tip. Many times the supercharged stuff liked ~10.5:1 and LESS timing. Obviously we can't beat and beat on a customers car while we are learning. The "burst knock" GM has in their code, must be there for a reason besides emissions.
As always, learning

The jet in the carby is mainly to fix the flat spot you'd get as there is a delay in the fuel from the main jets where as the air gets into the engine at the speed of sound.

With EFI its less of an issue as injectors react pretty much straight away as are close to the valves but its still useful as air is still quicker than fuel entering the cylinder so AE is used to compensate for that initial inrush of air but not to the same extent as carbys.

As long as you don't get a lean spike its all good and its doing its job. If you turn off AE you will see a lean spike most of the time if you are curious to see how much AE is contributing. Depending on manifold design, manifold fuel can be little or be a big contributor to AE as well, it's part of the AE calculation.