There is never a set rule for how much is lost because there are more factors than just how much you cut but you should expect at least 10 fps per inch.
Sometimes it is less than expected.Not a Disco, but it is a 24" .22 Crosman barrel and is run by air:http://www.gatewaytoairguns.org/GTA/index.php?topic=69630.msg662726#msg662726In the above case, was not a high powered rifle and had been set up to use little air per shot, which is likely why the velocity didn't change nearly as much as predicted. Also shows that the longer barrel was giving some "free" energy and used it's air supply a bit better.