[ConnClark]

The question is does smother flow on the under side of the boat tail really going to get a net gain by disrupting air flow some where else?

What I think you are trying to say is that flow around the wheels is already disrupted and that a bigger disruption there won't matter. In this paper they show for both the notch back and the square back that isn't the case as drag increased when they blocked off the transmission tunnel. If this is the case for your car I can't say and I don't think any one else will be able to unless they have a wind tunnel that can measure Cd to the 3 digit after the decimal point.