Gday Mark.
I bought a unifilter for my XT (
www.unifilter.com.au ) and haven't looked back since.
The factory paper filter used to let through lots of fine dust, but nothing appears to get through the oiled foam filter that I have now.
You need to clean and re-oil them regularly, obviously the more dirt riding you do the more often it will need cleaning.
No re-jetting is required.
Mine cost me roughly $90 australian. The cleaning fluid and filter oil set me back another $40, but I now have enough to last me for years....
They are expensive to buy compared to a standard paper filter, but they will pay for themselves quite quickly as they are re-usable.
I also use an oiled foam filter in my car.
I highly reccomend them....
Dan.