Lightweight yet more powerful bike?

[Pete7874]

I can see myself upgrading from the GZ250 to something more powerful (500-600cc) within a year so that it's more highway-capable, but similar riding position to GZ250.

However, I would like something that's not much heavier than the GZ250 (320lbs). I'm still not comfortable on heavy bikes. I was thinking of something like Kawasaki Vulcan 500, but it weighs close to 500 lbs, AFAIK.

So, does a bike like this exist? Any suggestions?

FYI, I am 5'7" and weigh 150 lbs.

Thanks!