With a properly done single coil ignition system, the timing advance has no effect on the dwell. The rpm is what limits the dwell.
With a properly done eight coil ignition system, the timing advance has no effect on dwell even when using one common signal to tell each coil when to fire. The rpm will have no effect on dwell either.
Maintaining an ideal constant dwell IS the main advantage of 8 coils, since that means you're providing the cylinders with the strongest possible spark at all times.
I predict you will run into this minimum coil dwell time somewhere between 4000 rpm and 5000 rpm assuming you're using around 36* of timing and you are switching to the next required coil at each TDC event (or each low-res leading edge).
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