I have a WP in mine and it was not the massive improvement I was hoping for.
You could get the shox rebuilt by someone like Maxton or a great suspension contact I have who lives just outside of York - but if I had the knowledge I have now, I'd just hold out and save the dosh up for an Ohlins - esp as it appears the SS/SL range 'appear' to respond well to raising the rear ride height.
Sadly even with GOOD discount you must be looking at £400+ for a new Ohlins however.
Frank