I have the six speaker arrangement and I have never been happy with the stock system. After reading several threads about this and learning more about the actual speaker replacement, I decided to give it a try.
Wanting to keep costs relatively low, I was looking for fitment and good quality. First, I replaced the dash speakers and chose the Pioneer TS-A878 and they really made an improvement. I then decided to change out the rear door speakers and knowing that the new dash speakers really brought out the highs, I went for a two-way 6X9 to get some improved treble, but solid bass and I installed the Rockford Fosgate R169X2, which sound great. Next, I decided to get two more of the RF's for the front doors and they arrived today. With the temps really dropping, I'll have to wait for some better weather to get them in, but I'm optimistic that the overall system sound will be like night and day when compared to the stock system.
The only down side is that I waited this long to actually do this.
Came across this and now see that I neglected to return to complete the commentary.
As previously explained, I installed the Pioneer dash speakers and the Rockford's in the rear doors and have since installed the same RF's in the front doors as well.
The overall sound is dramatically improved and all for a rather modest cost and much less time than anticipated. I purchased the Pioneer speakers through Jet.com for under $50 and both pair of the Rockford's from Crutchfield for under $150, including the adapter harnesses for free (along with free shipping). The actual swap-out took about two hours total and required no modifications, following the instructions from Crutchfield.
If you like serious bass, connect and install a subwoofer, otherwise the overall sound using the factory 8.4 system and the speakers mentioned perform remarkably well, even at highway speeds. Note: with my wife, we listen at volume level 15, but alone, I like it around 22 or 23. Depth is good and distortion very low. The $200 price tag and two hours of labor are well beyond worth it considering the virtual trash that came stock with the vehicle.
Note: Having the CD player gets better sonic results than listening to the frequency limited FM or Sirius/XM radio.