Few observations:
A refinery bigger than 2A is a waste of cargo space. A 2A refinery has 6 slots and you can fill it with 6 void opals, that's 2 more than a similar sized cargo rack.
Regarding pristine rings - i noticed that in a pristine ring an optimal yield generally gives 15 fragments. In a non-pristine ring i get mostly 9-12 fragment
Meh.
If you're flying an AspX, sure. A 2A Refinery will work fine.
If you're flying a Krait or Python (or anything bigger) you can probably fit a 4A Refinery without compromising your build significantly, so why not?
Heading to market with another 10 bins filled is another Cr16m in your pocket, if nothing else.
Not sure about the difference in fragments.
As a rule I just expect at least 15 fragments from each core roid, wherever I'm mining, and I don't recall ever being consistantly disappointed.
Obviously, though, I suppose that depends on proficiency with the seizmic charges to generate the optimal yield.
On which note, pro-tip for the OP::-
2 maximum-yield charges set into low-strength fissures will often give you the perfect charge.
If it doesn't, "top up" the charge by firing a couple of low-yield charges into high-strength fissures.
You
can, sometimes top-up the charge perfectly by firing one low-yield charge into a medium-strength fissure but that can sometimes be overkill (forcing you to disarm the charge and try again elsewhere) so it's probably safer to top-up the charge with 1 or 2 low-yield charges in high-strength fissures instead.