Whoah, when did I wander into the audio forum?
Generally I recommend against 4x10 speakers and recommend just cutting bigger holes in the back for 10's or 6x9s or whatever. That said, I do have a 4x10 in my dash. I have a crossover splitting the right dash speaker with this center dash speaker. My thought is that with only low frequencies coming from the center, the stereo imaging isn't affected.
I was thinking that if I have kids or a wife sleeping in the back and I want to hear some quiet tunes, that means turning off the subs and all the rear speakers, leaving only the dash (*shudders*), so any improvement to low end would be welcome. Now this has never happened, but it was a free upgrade anyway (already had 4x10s that I foolishly bought for the rear). Who knows, maybe I can actually notice the difference with all of the speakers on.
You could try running the Left channel to both 3.5" speakers and the right channel to a 4x10 (full-range) in the center. This might actually provide a better stereo image for the two people in front since it's hard to hear a speaker way on the opposite side. I have no idea how that will interact with audio from the rear speakers, but in your case they are really far away...
