If you look at your current downpipes, you'll see there's a small tube between pipes for cylinders 1+2 and the same for 3+4, those are the balance pipes.
If your current pipes aren't rusty, I see no reason to change them - I didn't. I used a high temp spray a couple of months back, but it's pretty crappy - I suggest the Granville paint that is further up the thread. I'll use that next time, and a tin should last me longer than a can. Although another (superficial) thing to consider is - do you want your bike shiny, or black? (I wanted black)