I think I mentioned this on the roadbike forum.
A very good Cat 2 racer (he can sprint with the best) would ride all Winter with a water bottle loaded with pennies. (he had it tie-wrapped to his frame and cage so it wouldn't rattle)

He claimed it made him feel sooo much stronger and faster come race time. (he removed the bottle just a couple weeks ago)

Anyway I am not sure of the actual merits of added weight, but I wouldn't want to be adding strain to my BACK by carrying it there. If I was going to "add" extra weight, it would be as this guy - Bruce does in a water bottle and/or with heavier wheels/gear.