Sonic difference K2CD vs SACD

I can only tell you that the redbook version is excellent. Not perfect but close enough.

K2CD and SACD versions being what they usually are, I would suspect an improvement of maybe 5 to 10% absolute max.

-IMO

It really depends more on the mastering than the format. You will not find a copy of each format with the same mastering to do this comparison. A redbook with better mastering will sound better than either K2CD or SACD. The 5 to 10% comment is meaningless

I totally agree with Ahendler. It would be great to have a general rule but it doesn't exist Mastering over format. I have redbooks that are better than there K2HD counterpart
I also have sacds that are not as good as the original cd. If you want the best sounding version be prepared to research the title and listen to different presses. I've been shocked at times where the better sound is found.