Heard that changing the rear rim size from 17' to 18' gives more high speed stability, wider range of tires and more ground clearance
Anybody done this with results?

Seems expensive and is it worth it, I don't ride rally's

There is nothing wrong with my current 17' rim

i wouldn't bother... since you don't have a use for it.

There are plenty of threads on this subject. It mainly relates to offering more tyre choice as you say but mostly it revolves around ridding yourself of that ridiculous safety bead and therefore being able to, more easily, repair/change a tube at the road side.

As for ground clearance = minimal

As for worth it = definitely if you are a trail rider and don't want to fight your rear wheel in 30C degree heat when you get a puncture. For those of us, like you, who ride where there is no tarmac / phone signal it's worth it.. However, it's directly proportionate to time you ride in the outback or where there is next to no recovery. It's not really an issue for tarmac types though.

As for high speed stability = Mmm? ..If it's unstable at 'speed', it will be down to poor tyre pressures/condition/bearings/loading.......blar blar

__________________
I have a lot of growing up to do. I realized that the other day inside my fort.