I have added screenshots to first post. While creating screenshots I have noticed that artifacts are visible in other areas too (not just when creating alpha override effect).
1st error is visible as a second outline in alpha override effect.
2nd error is that background text is outlined when gamma correction is set to on, but there should be no outline. I just discovered this. Background text is rendered as vector shapes (not with SpriteFont).
When gamma correction is set to off image is rendered correctly.