The UK site advises to put at least a 6mm washer above the rear shock, has anyone tried this??? I was wondering because they advise that it makes the XX handle so much better in curves.
Anyone??

Lots of us have the rear jacked up a bit. Do a search and you will find several threads.

Most suggest never going over 6mm. Search will give you lots of information. The rear of the bike will go up. If you are less than 6 foot, you may find it hard to pull the bike over curbs and bumps.
I did 4mm to start. It is easy to do and does help the bike turn in.
I think it was Warchild that posted a how to with pictures.

If you're handy with tools cut a piece of 1/4" steel in the shape of a U and slide it in after you loosen the top shock mounting bolt.
You be able to figure out the size by looking.
It really does make a difference.