I am using a yamaha headpipe. The kawi will proably make things worse as it raises the height of everything by 3/4"
It fit without the extension but not well. It was touching the manifold, and it had to be rotated in a specific position. There is no space to play at all. Adding the 1.5" let me rotate it to where it fit right in regards to touching the hull and to get it pointed where i needed for my waterbox. It also moved it away fro the manifold so i have a 1/4" or so of space between them.
I have no way to tell if there was any power difference with or without the extention, i never tried it before i added it on.