Draught-proofing and insulating first floor floorboarding

[Dancoll]

I have a number of related questions - answers to any of which would be appreciated!

I have bought a converted C19th chapel and the first floor has pine flooring which I hope to expose. Firstly - is it worth insulating beneath the floorboards - both from a warmth and a sound-proofing perspective (I understand that the main gain is from insulating the ground floor but is there any advantage to doing the first?)? Second, if the answer is no - is it worth putting anything immediately under the boarding to stop draughts from coming up between the floorboards? Finally, I have seen on t'internet adverts for things like gapseal - are these any good? They seem too good to be true!