I got mine to 150mph and ran out of gear, probably because it's +3 in the back. I wouldn't say it was wobbly or unstable, but it was definitely white knuckle for a bit. Just because, at those speeds, on the country road I was riding down, a deer could've jumped out 100' in front of me and all I would've been able to do is close my eyes and brace for the splat.
I guess I'm not even sure what unstable is though because my Z is the first bike I've ridden over 120 mph. My previous, and first bike, was a 1977 CB750F. You want to talk about unstable at high speeds, try tucking behind a 7" headlight and putting your feet on the passenger pegs on a 30 some year old bike just to see what she still has left in her.

After that bike, my Z feels as smooth as glass.
