I believe using the stock amp and stock head unit with 3 way separates, front or rear, won't gain you anything noticeable.
Going that route with a better amp will gain you very big improvements. But I would put them in the front not the rear doors. Rear is always for fill, that's gospel. Plus the labor on the door cards for the gain wouldn't be worth it to me.
The times that I have altered door cards for 3 way separates has always been for fronts, smaller two/four door cars and one or two tahoes. Rear fill cheapies in every install but good enough to handle whatever you want to send them. You want to hear it in your drivers seat, not the rear seat.
The placement of speakers in the LC front door isn't all that great. At ear splitting volume you can't tell so much, but with people in the car and just loud enough that people can talk next to each other you can. It's what I think about every time I drive now.
For the past week or so I've been debating 3 way components for the front door with better placement. Flush mount tweeters, and 4" mids somewhere in the center of the panel, 6.5" drivers in the stock location. But I was trying to stay under $1000 for the whole stereo.
At one point I drove around with just the new door (4) speakers and a JL amp, kenwood deck without the stock sub connected at all. You don't even realize it's gone. It's a waste of space, but plays a part if you're not adding any amp to the system. One little tiny decent amp will render that stock rear sub worthless. Driving four 6.5" with good power hits pretty hard, I bet that's good enough for most. A big amp will fit in the sub's stock location if you toss it. A smaller amp will fit opposite side forward of the rear AC unit behind that panel. A 400/4 JL class D will fit under the passenger front seat, directly rear of the NAV unit. Stereo stuff is my addiction in the used cars I buy.