The chickens don't end up weighing more at the point that they go to slaughter - the faster growth rate is so that they get to the same slaughter weight in a shorter space of time, which uses less feed. Chickens with faster growth rates therefore aren't replacing more than one slower-growing chicken.
The slower-growing breeds live for longer, which would be bad if it was extending the same pain intensity over a longer period of time, but this seems like it isn't what happens: https://welfarefootprint.org/broilers/