The stock 899 headers flow pretty well. When I built mine (early 2014) I had a friend build one at the same time. I used a full 1199 Termi and he used just the slip on. After mapping they made the same power.
Save a few bucks for yourself and just get the slip ons. You'll save a ton of weight and get 99% of the added performance. Use the extra money to get your light wheels.
For the record the stock wheels on the 899 are the heaviest I've encountered in a long time. Your biggest gain (of the mods listed above) will come from putting lighter wheels on.