All depends on how hard the leather is back there.
If it's really hard (which is what I'd now expect @ 11 years old), then you're either looking at patching in new pieces or grabbing new aftermarket leather covers. I created a how-to thread years ago on how to do this yourself too, but that will only be useful if the leather is still pliable.
The afterburners are incredible. I had them on my current 05, and passed them along to a good buddy of mine (BRZN) when I decided to make a catback change.
Those will only bolt up to the stock catback or the street screamer catback. You would need to just buy the mufflers from Kooks and have them welded into the Magnaflow catback if you wish to not ditch your existing catback all together.
The correct part number for those mufflers is R300-14 (3").
Otherwise, you'd need a stock or a street screamer catback to allow you to bolt those afterburners up.