There's only about 25 pounds difference in my K8 literbike and the k8 600, which has mostly been compensated for with new exhaust and wheels.
Some of the 600s I ride with are decently modded (within reason) and yes, there's nothing they can do on the straights. However, in the twisties, I can't push the 1000 any harder than a 600, the handling is the same. Only difference? I get cool points power-wheelie'ing out of the turns. I take my 600 out, we're neck and neck in the turns. On the 1000, same deal. Mine is just a LOT less forgiving. Pound for pound, a 600 is more fun in the twisties because you can really roach on them and they'll just stick there.
Remember, these bikes are made for turns, it's what makes them fun. Learn to love yours for what it can do, accept it's not a 1000, and move on. And no, don't bother upgrading. A bigger engine won't make you faster, it'll just get you hurt. My k8 is unforgiving as all hell. To be honest, it took 13 years of riding sport bikes to feel comfortable on it and really push what she can do, and I'm still not riding 90% on her (at least I don't feel it).