You can safely buy the most powerful 4ohm speaker you can find. Go for the high sensitivity ones, so that you don't need an amplifier.
I was planning to get infinity alpha or even 6532 reference, but due to my wife driving the car daily. Whenever I turn up the volume to compensate I get a scolding since my wife hates good sound, so I purposely leave her with the weak sauce 4 speaker setup in my E spec.
I'm pretty sure the head unit will not be powerful enough to kill a 30-40W typical, 130W max aftermarket speaker. Even if it can you won't be blasting at the power limit of a such a driver because it will be sounding horrible before it dies.
The important question is whether you wanna go coaxial (and lose the highs due to placement and directivity) or a component set which will require re-wiring here and there and potentially unsightly cables and a bulb of a tweeter. If you are with the V spec and above at least the front tweeter can be fitted at the A-pillar trim.
From whatsapp group chit chat most of those who upgraded to enthusiast level speakers say they improved from 5/10 to 7-8/10 with decent coaxials.
Whenever I drive, I suck it up and just endure the audio.
Let's say if you proceed to upgrade the speakers, would you upgrade all or just the front? Would you prefer to use component or coaxial assuming you're not adding an 8 channel dsp?