I'd prefer the slightest amount or none at all.
Both my match bats, Aldred Supreme and Tempo have only minimal concaving.
I've seen some pictures here where the spine is as wide as I am talented.
I reckon that heavy concaving is cosmetic and tricks the eye into thinking you've got a lot of wood where in reality it's an empty box.
Yeah, too much concaving can significantly decrease the performance in a bat IMO. I also have noticed that out of all my bats, although my Newbery Tour has the smallest spine, when tapping up a ball, there is barely any twist when I hit it near the edge, while when I tap up a ball on my brother's Richochet, there is quite a noticeable amount of twisting. IMO, the concave should never go below the edge, if you get what I mean!
I wonder if this is the case in a Dynadrive though... Anyone with one have any thoughts on it? Personally, I would think that the scoops in the Dynadrive work just like concaving, except it has the same benefits of a wider middle. Can't be 100% certain on this though, as I have only seen a really old Dynadrive!