I don't usually mix tyres (apart from on the Enfield which doesn't count!) although have recently been running my NC with PR3 on rear and Pirelli on the front as the front PR3 went flat morning I was leaving for a week away and I had the second hand Pirelli left over from my MT. Have to say I could really notice any difference but then the NC isn't exactly a sports bike. I have now put a brand new PR3 on the front to match the rear.
Like others have said, with price of pairs pretty good if you shop around then more a question of why would you. If you have a decent tyre fitter who will charge you a fair price then try Oponeo.com for great prices, a really wide choice and super quick delivery, have used them for last three or four years and not been let down yet.
