I'm not 100% certain that removing that rear spoiler will reduce Cd. We already know from the CFD images in the Mitsu VG study that the trailing edge of that deck lid (without spoiler) is in detached flow. The spoiler
may be providing an attachment point that helps things.
It'd be useful if there were Cd figures available for the car with & without it of course...
As for the other points mentioned, agreed! Aero is where you'll find your biggest gains. Well, driving technique may be as well, if you change your cruising speed from 80 to 60 mph
. On that note, glad to hear the first thing you did was order some instrumentation.