Nate, I think your dive shop misunderstood what you were asking. When you mentioned 300 bar the shop employee probably thought that you were requesting a 4500 psi air fill or perhaps a Nitrox fill.
I know what you mean about "religion". When the high priest speaks, the acolytes scramble to comply. Personally, when the robes make a pronouncement I am reminded that I live on earth, not in the clouds, or in a cave.
You are trying for a solution which fits your particular situation. Great, it's worth it, if for no other reason than an exercise in independent thinking.
The particular risk evaluations by you know who are pretty picky. When one of my old 3000 psi tanks failed I was presented with a free 3500 psi tank from the man himself, PST. Problem was it had one of those Euro style DIN valves which didn't conform to any of my other gear. I didn't want an oddball tank or valve. Before deciding on whether to convert my entire tank set to 3500 psi bottles I had to consider the valve type. I opted to convert to HP but to remain with yolk style valves.
This meant machining a number of suitable yolk valves to fit the new HP tanks. No problemo, they have worked fine over the past two years.
I've hardly met another diver who has any interest in examining how a valve works. Like you, I do, and I looked at some valves in detail. In 1977 Sherwood redesigned the yoke style valve face. They completely changed the dimensions of the valve orifice, the annular ring and the O ring. Instead of the old style compression scheme where the O ring was essentially a washer, the newer valves employ metal to metal interface which totally seals the O ring's outside perimeter. The inside perimeter of the ring is not sealed but that is not necessary as pressure forces the ring out, not in. Therefore, extrusion is eliminated as long as the regulator is locked down tight. Who needs DIN?
To answer my own question, divers who are worried about fishing line entangling the yoke screw. Also, cave divers who have 5000 psi in their tanks and bump their heads a lot.
DIN valves are OK, so are yoke valves. The differences are a matter of plumbing, getting the right part to fit the other part. The rest is theory.