If this lens would be purely for macro/product photography, would it make sense to just get a regular flash unit like the SB910 or a ring flash would do better?
Any thoughts welcome
[wince]
Ring lights may have advantages, but pleasing lighting is not one of them. They are merely a convenient way to get on-the-lens-axis very even light over the scene (i.e., abysmally flat lighting). Dull, uninteresting.
This could be good in some scientific uses, where the sole purpose is to show everything clinically and well, but proper photographic lighting needs a few shadows. Not harsh dark shadows, but soft gradient tones are extremely pleasant, and these gradient shading tones show the shape of the subjects curves, etc. Shadows are created by off-camera lighting angles, and soft lighting is caused by large lights. Like umbrellas for a studio portrait, etc. This is the basis of "lighting". Pleasant lighting.
You could use that studio umbrella for macro, at least in a fixed situation. It still offers the same advantages. Here is an example:
Four Flash Photography Basics we must know - Soft Light
But... macro work is very close, and close makes large. It is relative, so a 2 inch flash head at 6 inches is as "large" as a 2 foot light at 6 feet. It is easy to make it larger than 2 inches though.
So for a regular speedlight on the hot shoe, just aim it sideways into a small white reflector board (a foot or two size, and close, maybe a foot or so from the flash), which reflects soft off-camera light onto the little macro subject (try to prevent direct flash from hitting the subject - watch the shadows). A fixed setting could use a close reflector on both sides, with the flash aimed at one of them (and the other is fill).
Or put a white paper sack or cloth over the flash head so the light is effectively several inches larger (and camera close). A speedlight is probably six inches above the lens (off-camera above, esp to a six inch subject distance). Inefficient of flash power, most light may miss the subject, but at macro distances, it is of no importance. What does get there is plenty.
Anyway, the subject is lighting, and a ring light is NOT a universal answer.
Just one opinion.
And of course, the speedlight can be used for very many other uses too.