I disagree. Not nearly as effective as it has been on my other cars. This is not an opinion or a spreadsheet (my spreadsheet looked just like yours, BTW), I'm simply reporting my real-world experience which includes tire weight. As for reduced Moment of Inertia -- there is a theoretical gain there but it's not really noticeable in the data. The added grip of better tires is noticeable everywhere though.
Bottom line for me, the OEM wheel/tire combination is unusually lightweight. Including tire weight, you will save less than 4 lbs. by going to Allegeritas. Thems the facts . . . . Sorry if you don't like them.
I'll try to corroborate your fact shortly, but I believe it.
On other cars, there has been a significant weight savings in swapping to a lighter weight wheel of the same or even sometimes one size larger racing wheel.
So, there would be no more rubber, and possibly even less. The savings is in the wheel,
In our case there is a somewhat misleading weight saving in that the wheel is 10% smaller in diameter, but we're putting the same OD rubber on (so, as you say, a significantly larger and heavier tire).
Putting the lightweight Ultraleggerra 18"/19" wheels on may well result in a significant improvement over stock 18"/19" as the rubber is the same, but I don't recall the wheel weights on those.
Putting the Alleggrita 17"/18" on in place of the stock 17"/18" is also likely a weight savings, as again the tire is the same. I don't have the weights of those stock sizes handy right now either.
ALSO, don't forget that the P-Zeros are a bit "optimistic" in their tread width measurement. 205 is more like 200. So they are most likely going to be lighter than a tire that has a true 205 mm tread width. Plus, a number of people have gone to a wider tread with the new wheels, which likely makes sense from a performance perspective, but spoils a true weight comparison.