If your looking to save weight and have already striped the interior, the next best thing to do would be lighter wheels and maybe lighter body panels. I wouldn't start chopping up your body or you may lessen you structural rigidity.I really have no clue what speed wholes are either. Or if there some made up ricer thing. Just opening my mind to diff. ideas....If im going to reduce weight in my car and want to keep it my DD. Where could metal be taken out? Every thing behind the seat is gone already.
Good points..Actually, the HOLES in the front are generally there for more airflow to the radiator and/or intercooler and if done right are effective even at highway speeds.
The holes in the rear are there to relieve pressure that is captured in the bumper at higher speeds which will reduce drag. This is usually done on track-only cars and has little use on the street.
FYI- not all track cars do 150 mph, tighter courses don't allow that high of a speed, but don't you think aerodynamics have a role even at 50mph? Think about it. 
I'd think sealing up your front end would be better than cutting holes in it. A sealed front end gives less drag. If you think you can benefit from rear speed holes, just pull the rear bumper and see if it makes a difference (look at some of the nascars and you'll see that rear bumpers are much higher than the front allowing the air trapped under the car to escape). You can also close the bottom of the car up with some coroplast and that would probably be a better solution (check sign shops for coroplast it's cheap and you can cover the entire bottom of your car for under $50). Closing the bottom of your car will show better aerodynamics than the holes in the rear because the moving air stays stuck to the car and releases much smoother. Speed holes in the back will create a dirty aero wake, most likely causing your car to not see as much of an improvement. Link for underbody belly pan write-up with improved mpgs/ areo