Yeah, people are about the density of water but even an isle full of people isn't enough to be too meaningful to an airliner. The only times it may be a problem would be a short runway with a full fuel load at altitude. Airliner are typically flown well within it's performance limits. No airline would allow a flight plan that's close to the maximum performance of the aircraft since it would be a recipe for disaster in addition to hard on equipment.

Keeping the cargo secure and the wrench that extra passengers throws into emergency procedures is the real issue here.