I don't think it would add a lot of weight, but I wouldn't be surprised if it nearly doubled the weight of the thing. Haven't really dug into it, but the motors on these tiny toy drones seem to be based on the weight in grams of a machine, so even tiny additions are going to make a difference.
That said, I guess I recall having seen people say they are using the stock motors for their tiny whoop, so the upgraded motors might be more about speed too. I wonder if a nice controller (Spektrum or Taranis level) would make a difference in flying it. As it is, I don't know if the cheap controller it comes with has any fault or if it's all user error - but you are barely hovering, push the stick up, push it some more, and more and nothing happens. Then a tiny bit more and it goes to the ceiling. Pull the stick down it stays there, pull it down some more, still there. A little more and it drops slightly slower than a rock. Basically it's pretty tough to get a consistent altitude. It can be done, but takes a lot of fiddling around.