Just my 2 cents:
I wouldn't change the speakers.
Why is that?

Because the Infinity amp/speaker system is not that bad.
The components are not high-end but the integrated shark processor does some useful equalizing and the frequenca response is optimized for the speakers. The infinity amp only produces 22 watt per channel. If you want more control, you'll have to use a much more powerfull amp. But than you run in another problem, because the integrated Shark processor of the infinity amp does a lot of equalizing, has an x-over and it also seems that he does some time correction.

The main problem form my point of view:
The tweeters in the door panel and the bad sounding hu.

The tweeter positition is much to low. So you'll have a real bad staging.

If you have changed the hu (and that is a must, because that's the real bad part in the whole system) you can position some good quality tweeters in the a pillow and run them over the integrated amp. Don't forget to cut them off via a capacitor. Just add them via the integrated fader of the hu. Not to much, than you'll get a much better stage without sounding too much heights.

To improve the sound quality daramtically, just buy an after market hu with integrated eq. First step is to lower the frequency at 160 Hz at about -3 to -4 dB. This will improve the bass quality. No longer these "boom boom" sounds - especially in the coupe.

Also think about dampening the doors and the rear wall where the subs are positioned. This will also improve sound quality.

I attached some photos. The tweeters are Phonocar 2/428. They have an integrated capacitor.
The hu is JVC AVX55.

Photos show the car of german Crossfire forum member Emaxx. Installation was also made by Emaxx.

Added some dampening pictures.
Shows door dampening of german forum member cars sunraiser.
Rear wall shows my own car.