Mujitsu and Tairaku's Shakuhachi BBQ

Sorry for the cryptic subject wording -- I'm sure there's a better word for this but I don't know it.

In the context of hocchiku/chokan...  What's the general impact of the back-of-the-edge angle and is it ever considered desirable to deviate from dead-vertical?  i.e. if the utaguchi is cut at, say, a 45 degree angle, with the bore axis of the flute considered 0 degrees, then what is the effect of having a negative (as opposed to dead-vertical 0) angle to the other side of it?  Say, -5 or -10 degrees back into the flute.

Besides fattening the wedge of the edge (independent of edge "sharpness"), which my experiments tell me encourages a more noisy, breathy flavour...  is there a downside to a significant overhang in the fore portion of the bore?  I'm not sure if this is what's meant by a breath return, or if that term only applies to the rear of the ikegashi under the lips and chin.

This has come into play with a couple of big flutes recently, because the thickness of the node meat at the edge can be substantial so there's an option to lay the inside of the edge back (what I'm calling a negative angle) or ensure it's 0 degrees vertical (still with or without overhang).

Here's a badly drawn picture of the two ends of the spectrum... cross sections of front walls: (not to scale -- the real back angle I'm talking about would be much much closer to vertical, but the edge overhang in the bore could still be significant)

      /\              |\
     /  \             | \
    /__  \            |  \
      |  |            |  |
     b|  |  versus   b|  |
     o|  |           o|  |
     r|  |           r|  |
     e|  |           e|  |

Thanks!

-Darren.