Finally received my Arrow exhausts in the post today, fitted them up without any problems. First ride impressions are they sound great and the bike performs a lot better throughout the rev range. I do think it sounds and goes better with the DB killers removed. Fuel economy wise is a bit early to tell. I have set the Kev mod at about the 4 o'clock mark.
Here is a Link to a video on U tube with and without the DB killers fitted.
XT660Z Arrow exhausts