Some questions from my side now that 98 RON is locally available.

So as stated, finally we have 98 RON available locally. All it's life my 1.8 GTI (DX, Basic K-Jet) has been run on 95RON which up to now was the only available petrol on this island at the pumps. I assume that it was adjusted for it by the local dealers when new before being sold. I am now tempted to try it knowing DX engines are designed to use 98RON. Is it really worth the trouble in the sense that will I see or feel a noticable difference in performance, both power and economy wise? What kind of adjustments do I have to do apart from ignition timing? Should I do a gradual change or do I just fill up?

Yes do it… You need to advance the timing till the car pinks under load then back it off a little.
I found my car runs better on Super unleaded. Just fill your car up with it and after all the normal unleaded has gone reset the timing.
If you make a mark on the dissy shaft before moving the timing so you can go back to how it is now if needed. You will also need to adjust the idle speed as that may increase when changing the timing.
To advance the timing turn the dissy clockwise.
I find my tick over is smoother, I use less petrol as the car feels more responsive.

This may be all in my mind up it's worth a go.

If you do change let us know what you think?

I think by now most of the 95 is gone after 2 refuels. Have checked and marked the timing, and readjusted slightly, to bring it up at the factory markings. So far I only took it for a short spin. I think I do see some improvement though nothing outlandish I think. Will see how it goes in the long run…