I would not try to go any higher than stock gearing because in my experience it won't pull the gearing if there is any headwind or hills. I personally run a 14T counter-sprocket because I like the lower gearing for trails and twisty roads and I try to avoid the highway at all costs. It will still pull 70mph with that gearing no problem and have plenty left over. That engine likes to rev, don't be scared of cruising at 7,000 rpms.
If you plan on doing a lot of highway miles, I would recommend two things:
1. leave the stock gearing
2. look for a different bike, it is not the most suitable bike for lots of highway miles