I had rear speakers mounted in the deck and they sounded just fine. With the speakers you are running up front, you just need some rear fill. You won't need a lot out of the rear speakers to get the sound you are looking for. As far as being muffled, I didn't notice that much of a difference in my car, plus I had dedicated tweeters up front. By cutting holes in the rear deck, you will be gaining more tweeter if anything, and I don't know if that would actually make it sound better or worse. If you run enough power to the front speakers, a small amount of power to the rear speakers should be plenty. If you need it louder, I would buy a larger amp for my front speakers.