After reading all comments I guess there is no company who can Change the weak return spring with a better one?
On my 1994+8 I took off the seat belt retraction unit, pulled out the belt and put a couple of extra turns on the drum before letting go. This increases the tension on the spring, it's not weak, it's just not wound up enough. It's the same principle as the recoil starter on a petrol lawn mower when you change the cord.
