At this time, these are the only available configurations: http://www.ruger.com/micros/rpr/models.html
So no 20" in the 6.5, if that is something that a fellow wants. that said, it should be an easy barrel to have a gunsmith chop as it is a standard AR style barrel mount. And because it uses AR furniture, I'm sure you could shave a bit more weight by replacing the Samson handguard with something lighter an shorter as well as a lighter buttstock.
Come to think of it, you can probably replace the barrel with something with a more slender profile but then you would be getting away from the original intent of this rifle and might just be better off perusing something that already has a lighter barrel.