We certainly did.. the spacer kit i bought on ebay came with seal, o-ring and spacer all for about £14 or so. (Wheel spacers were another £14 or so).
So new spacer, new o-ring, new seal.
Have noticed a bit of difference now the Powercore exhaust is off.. the acceleration isnt quite as rapid. Didnt notice it at first as i was taking it easy due to the seal being blown when i took the bike over to eddies, but on the way home i gave it a couple of handfulls..lol. and yes there was a big noticeable drop in the speed of pickup BUT at my offroading level i was hardly using the throttles tope end, so expect it wont really make much difference. Might even mean the bike is a bit calm/less jumpy when i am off roading. Time will tell..