There are:
*squarebacks
*notchbacks
*fastbacks
*'aerobacks'
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Your car is a notchback which has the most complicated flow of all body types.There's easily a dozen body criteria which will impact the flow of their aft-bodies.
I suspect that Honda has already 'optimized' everything you see in their wind tunnel and the introduction of the partial Kammback could compromise what they've accomplished.
If you had a hatchback,you could remove it altogether and construct an ideal Kamm roof profile of full length.More like what basjoos has done with his AeroCivic.
For you to do a proper Kamm roof as you've learned,will cost you access to the trunk.
Short of removing the trunklid,fabricating a overarching enclosure,you're kinda forced to do an articulated,hinged Kamm addition which can flip out of your way to get to the trunk.
The partial Kamm I'm afraid will never give you the performance you seek.
