Here is some info that may help since you have the 6 speaker system. Can compare to what is equipped in the limited trim:

 

I'm going to make a post about my audio upgrade whenever I get a chance, but long story short: I replaced every one of the 6 speakers in the car, installed/tuned the Beatsonic DSP, added sound deadening, and installed a sub. My verdict is that the rear speakers are the least important and I would not have 'wasted' the time upgrading them if I wasn't already removing the panel for the sub/deadening and if I didn't already have the speakers bought for me as a Christmas present. Adding a Beatsonic amp will be both easier (in my opinion) and make everything sound better than replacing the speakers. I would also recommend adding a sub before upgrading the speakers as well.

That's just my 2 cents though, so do whatever you want. I installed Polk DB402s with a hacked together bracket made from 12" straight braces. The harnesses you'll need (at least for the 6 speaker system) is the Metra 71-8105

 

I just grabbed a pair of Kenwood Excelon KFC-X2C rears. Bought my GR86 used and of course the right rear speaker is blown. Person before me probably was overdriving the system (GR86 base; 6 speakers).

 

I share your feelings about the surround sound feel. The factory setup seems to have the rear speaker volume so low the music sounds like it is coming from in front of you. It is like you are in the back row of a concert. I recommend going with one of the Beatsonic amps before changing the rear speakers. The Beatsonic amps have a separate gain for the front and rear. This allows you to increase the rear gain and bring the focal point of the music more rearward. I suppose you could pick a rear speaker with a different impedance or sensitivity and get more volume, but replacing the rear speakers was a major pain. Not much is standard procedure back there. I have replaced all of my speakers and installed the Beatsonic DSP amp. I added 5 dB to the rear. It is a much better experience. I am more centered in the music now.