Instead of butchering your rear bumper and allowing air to (presumably) flow through it, you should consider building a rear belly pan. Look in my albums for a diagram on belly pan departure angles.
But yeah, belly pans are a pita. I ended up starting the Probe's boat tail with a belly pan starting at the rear axle. I figure the air dam pushed enough air out of the way to make the front and middle of the underside quite unimportant.
If you're making a pan for a commonly accessed place like the engine bay, make sure it's easy-off, easy-on. I used zip ties and a few curse words... not fun.
PS- x2 on removing the air tabs from at least the rear of the car (why would you want vortexes there anyway?). Another user has shown EK sedans to have mostly attached flow on the rear window so whatever gains you might have there would also be negligible. Without proper testing procedures, 0.5mpg difference could be due to wind, traffic or other factors. Too small to really claim "gains".
