Volume isn't really my concern. Quality is what I am after.
Are you looking for true sound quality where the speakers completely disappear as far as locations sound is coming from you cannot tell and it basically sounds like the band is on your hood, center image center of dash, good separation from instruments/singers. Basically be able to close your eyes and point to where the artist is standing. The complete sound stage is in the windshield about eye level and from one rear view mirror to the other in width.
If so you'll want a DSP running active, quality 6.5 speakers for front doors and a larger tweeter (1.5" or so) for the a pillars that will be able to play lower then the smaller tweeters and blend with the mid drivers. The biggest key will be tuning of the DSP, which will make or break a system. There's some nice DSP/amp combos available but not cheap. There's also a few guys that tune those DSP/amps remotely with phenomenal results, prices vary for those services as well.
I'm a huge fan of Helix DSPs and run a Helix P6 mk2 that powers Audiofrog GB15 tweeters and Audiofrog GB60 mids running active (each speaker has its own dedicated amp channel which gives you extreme control with EQ for each speaker)
I've heard good things about the kicker key I just don't have any personal experience with it. I think it has a self tune option which is great if just getting into this slippery slope rabbit hole hobby.
I still think grimreapers option would be a solid choice for a much improved sound system over the factory without diving so deep into this.
Edit: grim was too fast for me haha. Sounds like the kicker key is a solid option at a very good entry price