after some more testing, and a 6 hr trip, 1/3 of which was city, I had put the upper grill block on and removed the lower. Ive decided the upper grill block is the one that is more restricting to my airflow and is the cause of the bubbling in my over flow. plus, I think it isnt as affective of helping create a more aero front end.
so I removed it (again) and put the lower GB back on.
I have a trip planned to Orlando (about 2-2.5 hrs 1 way) to pick up a set of OEM side skirts, which Im hoping will help to clean up the flow around the sides. plus Im thinking it will give me something easier to build upon if I want to enclose the underside partially.
although looking at the belly of the mp5, its pretty clean (or Im just too used to trucks....)
either way, I am going to put a front pan on, while it is pretty clean under the front there is more than enough room for improvement
also of note, my 2nd tank of gas was all city/town driving, and I still managed to net 29mpg, and that will only improve as I have an SG ii on the way !