If you guys are comfortable, then that is all that matters!
Your bike will be a lot easier to handle on rough roads, mud, sand than Grant's big and H E A V Y BMW.
If you're doing 300 mile days then your GF is a tough Cookie. Damn tough!
Now be prepared to do that every day, for months, in bad weather and in dangerous traffic. But the G650 is about the best choice you could make.
Easy to ride, nimble, relatively light, reliable (mostly!) and most of all ....
FUN!
I would study up on what to expect from your bike on a long ride maintenance wise ... but whatever you do .... don't read anything from Smelly Biker or Maria and Guy. If you do, you'll never take that bike out of Oregon. Yours is the newer "G" model Chinese version, but reports are pretty good. Watch wheel bearings and broken shock mountings.
You may want to beef up the suspension to handle the luggage weight. I would. Careful of the wheels too ... they dent pretty if you hit something really hard.
Read up on Colebatch's thread over on ADV about the BAM road. His buddy rode a F650 single, similar to yours. One of the toughest rides in the world.
A fun thread for BMW fans.
The BAM Road - ultimate test of man and machine - ADVrider