My opinion: I've seen a lot of people drag race with extension blocks, I think they are safe enough when properly installed and checked regularly.
I've also owned a +6" ZX6R and went on some fun rides through the Ozarks...and smoked everyone I rode with. Now, I will admit that the bikes handling changed after the stretch, but it didn't make feel scared or uncertain as to it's ability to stick a corner. My only real complaint was the turning radius. It does take a much larger half circle to turn around. It did look sweet and I could still have my fun in the corners.
However, if your are going to try and drag off your knee pucks in a corner I would advise you to leave it stock length. Valentino Rossi doesn't have extension blocks on his ride, so if you're going to ride like that then I can't suggest this mod.
---That's my two cents...