The first header that I built I supplied a reducer to step the 2" Tail pipe down to the stock size of 1¾", this is removable.
I built these headers like this as there was a lot of aftermarket rear end systems such as the DP systems out there.
This header with reducer was a direct replacement for the stock header.
This would plug straight in to the stock CAT/mufflers etc (all models)
When I first got my bike I dropped the CAT and three mufflers out.
I made a prototype cross over pipe with two mufflers.
This made a noticeable difference to the performance of the bike.
Some time later I fitted one of my headers (with the reducer) straight into this free flow cross over and mufflers.
Also fitted K&N Pods at this time.
Along with a tune to suit.
Now this really woke the bike up. Amazingly (to me) the exhaust note was actually quieter

The performance was seriously improved, a whooooole heap more fun

I'm working on a system now that will eliminate that 1¾" restriction, that I'm sure will make a noticeable improvement.
Cruising on the open road I get 40MPG, blasting through the hills I get 26MPG
and it is tuned correctly !
Overall, fitting the header (along with the K&N's and tune) made the most significant difference.