Pretty much if you plan to run speakers off a head unit I would try to find a pair with the highest sensitivity rating you can. Basically the sensitivity rating is done by powering the speaker with 1 watt and measuring the db it puts out. It's a rating of power efficiency. I wouldn't worry about the rear ones. Just worry about the front sound stage. Upgrading the rear is worthless due to their location. Those Bostons for the front are nice but I would try to find something with an amiable tweeter.
These are some other options:
Infinity Reference 6032si 6-1/2" shallow-mount 2-way car speakers — for 6-1/2" and 6-3/4" openings at Crutchfield.com
Polk Audio db651s 6-1/2" 2-way shallow-mount car speakers at Crutchfield.com
Infinity Reference 6022si 6-1/2" shallow 2-way car speakers For 6-1/2" and 6-3/4" openings at Crutchfield.com