IMO skip the $ and time spent on upgrading rears. Just run them off HU power and fad your HU so the 'stage' appears to come from the front.
I'm running these guys off my Pioneer:
They have a recommend range of 25-200w with a 89 dB sensitivity. I just have them crossed over @ 63hz so they aren't getting too much bass on stock Pioneer power. Still plenty loud/clear. Plus I have room to grow when I finally get around to adding an amp.
Those polks are even more efficient at 92 db, so they'd be great for Aftermarket HU power or better yet of course with a small amp.
And dam myself for looking at those Polks, I'm thinking I might have to swap my CTXs for them Polks. They should definitely sound better in my current setup.