Ok i am thinking about purchasing an aftermarket header but before i do. When installing the header is reflashing or a new ecu required? i figure not but i just want to make sure..

 

No, It is a good idea to reset the factory ECU by pulling it's fuse or disconnecting the battery.
One reconnected let the car idle for 2 minutes

 

The stock header is pretty effective so you will see minimal (if any) change with an aftermarket unit. You may want to consider using those funds elsewhere.

 

also with a aftermarket header, you might want to get lets say a apexi i vafc to tune it a bit. The oem header is very good, a bonus to swap it out is for weight savings, usually about 10-15 pounds lighter.

best of luck with your choice