That's interesting. I am retro-fitting a 6 gallon tank to my Evo in the expectation I will get more mileage out of it! Mileage on the new tanks has been covered before here on HDF and it seems to me worth riding until the bike is seriously low on gas, just to find out the true limits of the tank. One contributor got over 6 gallons in his while running on fumes.
I also own a BMW and I recently read on one of their forums about a tank mod to one model which claims to increase tank capacity by over half a gallon. This involves drilling holes in the neck of the tank filler, to allow air out and gas into the upper-most part of the tank. We may be able to adapt that to these Harley tanks.
Pics are of the 'top plate' that hides under the console of every 6 gallon tank. The fuel pump and other stuff is installed on it, but as my bike is carb I don't have any of that to show you. First pic shows the plate with filler cap from above, second pic shows the bare plate from underneath. It is a single plastic moulding and the white parts you can see are an electrical connector and venting valve.
The 'BMW solution' I mentioned would drill 2-3 holes, say 1/4" diameter, in the filler neck close to the underside of the plate. They would best be in the gaps between the strengthening ribs you can see in the second pic. It may be a while before I get my own tank project complete to test this out, also I don't have any current experience of riding with it, but the more adventurous of you might like to see if it is worth doing!