I echo what Ando said. For optimum performance always use the largest carrier that doesn't produce a leak without any shims installed. Are you using level 10 shims or ULT shims? I'm actually surprised that your setup doesn't leak with 5 shims installed so I am assuming they are thinner ULT shims and not the thicker level 10 shims.
When testing the effectiveness of the level 10 with your finger, make sure you hold it against the bolt face. If you hold it more than 1/4" from the bolt face, the bolt will hit with full, level 7 force and it could hurt.
Changing to a longer stiffer bolt spring makes the level 10 softer on paint. You should be using the bolt spring that allows you to shoot at a velocity that is about 20fps above the lowest velocity that the gun will shoot using the same bolt spring.
Are you getting chops or barrel breaks. Check the inside diameter (id) of your barrel and also the balls at their seams. You should be using a barrel id that just touches the balls on two sides opposite each other at the seams. If the balls are brittle and the barrel id is too tight, then you can have random breaks when shooting. A level 10 bolt won't help with that.
Except for the Automag in front, its usually the man behind the equipment that counts.