The lease will be hard to enforce, and it will cost far more money in legal fees to do so, than the cost of paying the floorboards to be lifted and relayed over some acoustic matting.
And as the construction is existing, I can't see how it could be proven to be a nuisance legally.
But I don't think this is an argument that says the costs should not just be split 50/50, you both own the freehold to the *building* which the modifications will be made to.
You either make the modifications and go 50/50, or don't do them.
That's my completely ignorant legal opinion.
Morally and in regards to common sense, I would also say just go 50/50, the bottom floor may change hands, and the next people may not be so accommodating, may turn it into a noise war, also with freeholds there is the chance that something else may need to be done in future, and its best not to establish a practice of arguing over costs, anything to the building fabric or grounds should just be split 50/50 to avoid headaches.