Ah, you've discovered that this most common of "home theater wall" designs doesn't work too well acoustically. Read the article below which tells how to do an installation in a more acoustically correct manner;
http://www.audioholics.com/techtips/roomacoustics/roomacoustictreatments.php
In answer to your question about your Mirage center with the rear firing drivers, the answer is yes, it should be replaced if you intend to keep your home theater wall. Actually I wouldn't ever recommend anything but a direct radiating speaker design for a center channel since this is where at least 70% of movie dialogue comes from. A direct radiator works best for speech intelligibility.
You've discovered that moving your left-right front speakers out to the plane of the television works much better. Good, you're on the correct path. You now have only a couple of options to settle the WAF.
1) attach the left-right speakers to the side of the prominent TV cabinet so that they are at least on the plane of the front of the TV cabinet or
2) Run some type of support left-to-right or hang the speakers from a ceiling support.
Option 1 will severly limit your left-right LCR spread which is recommended to be between 22.5 to 30 degrees from center on either side.
Option 2 will work really well, much like you see with my Modulus set-up in the article. A couple of notes on the Modulus>
a) the support rod doesn't really bend as it appears, that's the barrel distortion of the camera and
b) my LCR spread is only 20 degrees (and I sit only 8 feet away from my 40" Misubishi. I'm in the process of changing this system out for some other satellite which will allow me to get larger LCR spread to expand the front soundstage.)
Finally, once you've got your LCR compromise down to something acceptable to the wife, take a look at adding absortive material to damp out those first order reflections. Again, refer to the article above for details. Good luck, this already-built design is a very tough situation to rectify to a more acoustically correct layout.
