I have had a k&n filter in my roady for 5 years with no problems, I have only cleaned it once, great sound, a nice sucky noise and it smooths out the gearchange a little, that is why I changed it from stock as I read it smooths the change, and it does, but smartmadness sound as if they know what they are talking about, (wish they would open a branch in Sheffield), so I would get in touch with them and get their opinion.
Get it done it is a great cheap mod and should be the first thing you do after buying the car.
Keep the old stock filter and change it over if you have to take the car into the dealers, takes 30 secs.
Ellis