While I can not disagree with your theory, I believe that small amount of tire weight is splitting hairs. I also understand that it all adds up and you usually have a full load. 20 pounds, 5 pounds per wheel is unsprung so its not a load on the chassis, but is on the driveline, just the same as it would be if you had 20 pounds more of Coors Light in the cooler in the back than one normally does. But with the cooler in the back, now you have the chassis load AND the driveline load. I think one could have 10 pounds of weight per tire just based on the conditions that come and go 10 times over through the course of a ride with things like mud sticking on them. It is also probable that a tire with less aggressive tread will slip more in many conditions nullifying some of the effect of the increased weight. Again, splitting hairs. The thing that's gets my attention is the $200 less cost for a set of 4. I can just about buy a set of aluminum rims for that price, which will also offset the tire weight and look cooler!
Just thinking out loud here, if you have a 27" tire and a 28" tire and they weigh the same, which one if any creates more stress to the driveline? Does the size of the tire (traction and rolling/braking resistance comes to mind) cause the stress as much as the weight would or would not? We could probably go on and on and use up all 20 pounds of the previously mentioned Coors light if we ever got together to talk about random things like this but.....