I've set up my suspension pretty good with the stock 16/38 gearing and chain, went through all steps of setting the sag and adjust the dampening step by step until it had loads of grip and feeling on both font and rear on the track and road.
Lately I had to adjust the chain tension twice because the chain was wearing out and I had to compensate for the longer wheelbase by turning the rear compression 2 clicks in (more dampening). Bike felt like it did before so all good.

After changing to a 520 chain with 16/40 sprockets I had to reverse the adjuster blocks thereby shortening the wheelbase by a couple of centimeters.
I opened up the compression by 1 or 2 clicks to compensate for this but the rear still felt a bit too stiff so today I opened it up another click and it didn't feel overly stiff anymore. When I stand next to the bike and push on the bike both sides move up and down simultaneously so it's balanced again.

So far, so good but in corners the rear feels like it wants to slide or already is sliding, even with less than hooligan speeds, almost as if I sit on the pillion seat (which scares the crap out of me because I think that feels like it's sliding all the time).

Edit:
Previous time when I had this feeling I gave it an extra click of rebound dampening on the rear so I've done this now. Will have to ride the bike to test so I'll get back on this later.

Surely one click of rebound is not going to be the difference of the rear sliding in a corner and "rock solid planted"?

It doesn't feel like it's trying to overtake the front or feels like sliding every corner. I know I'm really (according to some overly) sensitive to a bikes behavior so it's possible other people won't really notice it and I really don't want the rear to stack. Last time I had a sliding feeling on the rear I gave it 2 clicks which was 1 too many (stacking) therefor I only gave one click.

If you can feel one click then sounds like you know where you want it to go, or feel where you want it to.

Try letting your preload of at the shock