has anyone replaced or upgraded his gearbox, I'm not really happy with current (4.0 MK1) gearbox and might look for a close ratio (or maybe the S-version is any better?)
Might be the diff is too long; gear box should be fine on a non-S Mk1. What is it you don't like about it? If you go CR box then you probably want to change the diff too.
Indeed upgrading to an S is an interesting option but then make sure it has a CR box (not all S's have them).