So the last like, four months, I don't think I've busted out a mid more than ten times (aside from a round at Phantom Falls where the drivers rarely came out) and instead have been using putters. I've gotten to the point where I'm wondering if I even need mids for the courses around here.
Conversley, I'm stuck at about 380-400 feet max D. So I've been using my Volt and Teebird for most drives. It's really been helping my game, and I feel a lot better physically because I'm not putting so much effort into the wide-rimmed stuff, which remains in my bag because I'm too lazy to take it out.
Point: I think it's great that you're getting such an Understable disc out that far. It shows that you have pretty good form because it's not turning and burning on you. I would say perhaps you are putting too much effort into throwing the Boss, trying to "muscle it" up to speed, which is why it's flipping on you. A Roadrunner is so much more flippy than a Boss, and if it's not turning and burning on you, I don't see why the Boss would.
Back to the original intent: in theory, a hyzer-flip of a RR would yield more distance, but as others have said, you're getting it out there pretty far anyway, so I'm not entirely sure it would help. Maybe experiment a bit by releasing it on smaller hyzer angles (I learned to hyzer flip by keeping my arm and shoulder straight but keeping my wrist at a hyzer angle, if that makes sense.) I've seen people throw Archangels 450+ feet (albiet rarely) so I think you could get the Roadrunner out there. Half the fun of disc golf is the struggle to break the plateau. Good luck, and let us know how it works out "fore" you!