according to a 2013 press release, the goop only adds a pound per tire: http://www.tirebusiness.com/article...ealing-tire-technology-could-be-a-gamechanger
but this article talks about how they get the tire to have low rolling resistance. which may mean a heavier tire to get it stiff enough:
https://www.wired.com/2016/05/hidden-battle-make-perfect-tires-electric-car-divas/
"For the Bolt, coming later this year, Michelin delivered what it calls its best tire ever, at least in terms of rolling resistance. That’s the tendency for a tire to deform, requiring more energy to keep it rolling. The less energy you expend there, the farther you go on a charge, making a stiffer tire optimal. The tradeoff, though, is comfort."