So after looking around at the options and the majority of music I listen to, I decided to go the BSW route and upgrade the front sound stage and add the sub. The rears get to stay stock since they are predominantly fill.
How did it turn out you might ask?
Short answer, very well. I could not be happier with the results. Dramatically improved depth and crispness and the sub works perfectly for my tastes. A 10" would have been overwhelming.
Long answer. There were some hiccups, but that's to be expected when you start opening up your perfectly good vehicle doors.
The installation went smoothly with a couple of minor exceptions. The speaker cable was somehow missing and being "the" Saturday available to do the work, I was not about to call BSW for a short piece of speaker cable. Sufficed to say I have done a few speaker projects in the house and the car so I had some left over. Second, the mid ranges in the dash don't quite fit the existing mounting holes perfectly. It takes a bit of creative srewing to get them in place. Aside from these minor bumps, getting everything in went fine. The BSW supplied instructions could be more detailed but they cover the bases well enough for anyone who's willing to tear their X apart. I opted to make the install slightly more challenging by adding some sound deadening material (Dynamatt) but it was easy to place and definitely was worth a bit of stickiness!
Performance - The good stuff! As expected the front stage is much improved. The highs are crisp again and the depth is noticeably improved. Add to this the low end from the sub and I am very pleased. They have been in for a week now and I'm still tweaking the amp tuning but that's fallout from listening to everything from Bach to Usher to a little George Strait. I'll post again after I get things dialed in but so far I am more than satisfied. I have no doubt I will get my money's worth from the upgrade!
(And no, I do not work for BSW, I'm all the way across the country from them!)