I did some testing a while back with a now defunct software program and I could never get the Cd figures anywhere near the ballpark of where they should be ( user error of course )
One of the tests that I did was to compare A full boattail, closed off kammback, open kammback, and a kammback with a box cavity.
Despite showing Cd counts
much higher than they should have been, there was a consistent .1 count in the Cd between the open box cavity and fully open kammback and the enclosed design.
Figures were most likely around 10 counts off ( ! ) but showed a Cd reading of . 36 versus .37 for the closed version.
Based on that, I am going to say that an open design like you have will be more beneficial than one that is sealed.
Fully closed off :
Box cavity :
Fully open :
