No need for more fiddling!
I'm not sure how the undercarriage is in real life but I doubt that lump is there. Anyway, here's what I suggest. When you can, do tuft testing on the rear window to see if you have attached flow. Flow Illustrator says you should but you'll want to know for sure. If you do, leave it stock without any spoiler. If you don't, add the little Kammback thing I threw on there.
In either case you should consider a full belly pan with the rear pointing up just about 5* or so.
Good luck tonight!
