I can't answer your question, but have one of my own: How do you like the 310? I'm looking to downsize from my 1999 R1100RT, and the G310 R, not GS) is on my short list.
Well....my normal ride is the '14 R1200RT. I affectionately call the little '18 G310GS "thumper" because that one little cylinder really does chug/thump and plug away. I'm heavy at 250 and about 5'10" the G310 works hard to climb the hills with me on. The front springs are exceptionally soft and compress way too much when I brake. I upped the pre-load which helps a bit on the main shock.
I've only ridden about 250 miles and to tell you the truth the jury is still out for me. The seat position is very far forward which requires adjust if you're used to the RT. It's nimble and light. I'm still getting used to the diminished power as I use it to commute on here in CA. I have to make sure I have plenty of running room before I jump into traffic.
The g310 would be BIG downsizing. I would transition to a 650, 700 or 800 before dropping all the way to a 310. Not sure if that helps but it's just my 2 cents.