If a 5.8:1 Alphas/Sol, etc. take in 23ish inches per turn with an 80mm handle, what is your best guess as to what a 100mm handle would increase that figure to? If you've done it already what was the result?

It would increase IPT by nothing. Handle length has nothing to do with the equation. It's gear ratio and spool circumference that affects ITP.

As stated handle length will not change IPT. If anything, a longer handle will actually cause you to retrieve less line in the same amount of time vs a shorter handle at the same speed. The longer handle has a larger circumference and therefore it would take you longer to perform one full rotation due to the increased distance your hand has to travel.