I've had this similar thing for almost 2 years. I started doing a ton of research, talking to local pros, etc, etc. One thing became obvious. Arm speed will only get you so far, you need more spin to maximize distance potential. I've been doing long term field work with putters. A by product of improving putter distance is you will also "automatically" develop more snap, spin, etc. Because to get a putter out to 275-300+ it's not just about arm speed, putters need that spin to glide far. So here's the thing.... by adding more snap to my form, that separation in distance from mids to drivers finally happened. My drivers now travel a minimum of 50ft further than my mids.
I'm an old fart, but I also routinely do field work with a local young gun who can now throw 550ft plus. The same thing applied to him. He was plateaued around 400ft or so. We worked on putter throws, he started throwing putters out to 360-400ft and then all hell broke loose and he now throws drivers 550+. His arm speed was fine, but he just didn't develop that natural snap to spin the disc more for that extra glide. I totally agree with people that say "snap happens automatically", but you can really dial it in (quicker?) by working those putters into your field work session and keeping track of how far you can throw them.