I'd suggest avoiding the stock 900RR header if possible, it will be a flat spot in the bike at a certain RPM, but that is really not a huge issue - it is there though, dyno proven, long ago -
There are so many options in "real" 4-1 headers, go aftermarket and get the best you can find, or, do what I did and try them all.
Here is the one I sold the bike with - a stainless 4-1 header make by Borla - found it brand new on eBay for $99.00 (header only)
Here is a Kerker system I bought, these are out there, and it seems to perform better than the stock 900RR header -
Never did make the 929 header work, angle seemed wrong - anyway....
Full Kerker System installed -
Here are the 919 and 900RR headers compared - those "expansion pipes" in the headers, I'm convinced those are what caused the flat spot on mine -
Headers, low pipes - I tried a few -
Here is a very early 900RR full system, I thought it really looked good -
And a later 900RR header with Yoshi canister -
Kerker header with Yoshi can -
Fun times those were.