Can anyone say there are real differences in brands w/ the typical 1/16", 1/16", 3/16" ring packs? Ya' got Speed Pro, Total Seal (talking about their conventional stuff, not gapless), the piston companies like Diamond and JE, et al.

Is it basically a case of finding a supplier for the correct bore & oil tension, as opposed to one brand having some discernable functional advantage over the others?

there are definite differences. keep in mind that many brands are re-boxed, but even among those, some will still use a particualr brand/style of oil ring that another brand may not. so in those cases, the top and 2nd ring may be the same ring, but the oil ring can make a big difference.
also, some brands use a moly coated top ring on a cast ring, other more expensive rings are plasma moly on a ductile iron top ring. i could go on and on.
as far as piston rings go, in most cases you actually do get what you pay for. there are top selling brands that are far more expensive thatn their "budget" competition. nevertheless, they are still the top selling brands and for good reason.
keep in mind that minor differences in design parameters are sometimes used in the top and 2nd rings for some companies that re-box also. like i said before, i couold probably write a short novel on this one subject. and to think that all the different types and styles of rings on the market today are made primarily from only 3 different companies.