How much more distance do you actually get with a full spool versus 1/8-1/4” gap from the lip?

   A fair amount ......... just before you get the wind knot and have to cut 20 feet of line off.   ☹️☹️  

   Been there, done that.        jj

Nothing good comes out of a spool filled all the way to the top

It depends on the reel. Not all are created equal. I've had 15lb PP Super Slick on my Tataula LT 3000 CHX with the spool filled right up to the chamfer on the spool's lip for 2 years and haven't had a single wind knot. So try it out first with "your" reel and see what happens.  Spool lip friction kills distance with light baits. You want as much line on the spool as you can get away with. Your reel will tell you what's what.

The effect is magnified if you have a leader knot

The real question is do you get the extended distance long enough (time wise), to make it worth it, given that sooner rather than later you're going to get a wind knot have have to cut maybe up to 20 yards of line off the spool?

 

Me, I fill spools up to what I think is "right" and every so often I still get a wind knot.  I've found that the lighter the braid, the more often I get wind knots, especially when I'm throwing cross wind or quartering into the wind.

 

I've found that the "right amount" of line varies according to how many IPA's I've had when I'm in reel spooling mode.  That is unlikely to change.  I've realized that I am very positive and optimistic about how much line I can put on a spinning reel after a few Boulevard Single Wides.