This is definitely the sort of thing you'd want to gather data on. It's plausible that it could save Terabytes of bandwidth per day at Facebook/Google scale.

That's like saying a car wash can save 3 drops of water at the end of the day.

An extra character will cause 1/512 of responses to take an extra packet, so the amortized cost is still one character per response. Presumably this matters at scale.

Not if your average response is less than 512.

If your responses are all between 505 and 512 in length then it might matter but most likely you are prematurely optimizing.