To me, Swiftstrike Buckler is the optimal shield. There are very few cases where I'd rather use something else (SLs, DMs) and since Swiftstrike provides ASI High, I have no good reason to get more than ASI High unless we're talking about PvP gear.
Any gunslinger or swordsman should reach a point where they very seldom need anything but Swiftstrike so I'm wondering, how worthwhile do you people think getting ASI VH instead of High is?
One notable case where a VH UV would actually make a difference is if you have one piece of ASI gear, giving you a max and allowing you to use any shield of choice or BTS for greater sword damage. Still, one should still consider that getting the most bonuses out of your gear means using full Chaos/Chaos+BKC and thus, one would gain more from Swiftstrike unless one has an ASI VH UV.
It's hard to know what to take as "given" in your question. The answer seems to depend on what other items, UVs, etc. have already been fixed. But I think that your main point is: Abilities usually come in increments of +2. Thus either SSB's extra +1 over BTS is wasted, or ASI+4's extra +1 over ASI+3 is wasted. I can't think of a counterexample (for end-game players).