Firstly I would inspect the crown to see it has been recut correctly , the smith who undertook the work to thread the barrel I am sure would have done so if he knew what he was doing . also to be considered were the barrels cut in the right position ie length . It is said Anschutz barrels and I have heard this of CZ as well have a tight spot at the muzzle and if this is cut off the benefit is lost , although another sweet spot may possibly be found in the barrel .
Also inspect the crown to make sure it is clean and even , smooth , test by wiping a peice of cotton wool around the crown and see if any fibres get snagged , you can polish the crown with a round head brass screw placed in a drill chuck and rotate with some polish around the crown , you can find and see this type of info onthe web .
I would not test the accuracy of the rifle with the mod fitted to establish where the problem is , rifle or the mod , it could be the screw cutting is not perfectly aligned , which will cause the the bore of the mod to be out therefore causing damage to the mod as the bullet passes through , and it is clipping the mod at some point .
Hope this helps .