I did not intend on writing half a book on the matter but it turned out that way. You have been warned.
Fitted Focal's ready kit to the front yesterday, soundinsulated doors completely (every bit of the door metal). (Non-JBL, Circuit pack car)
Now the results are obvious, the OEM speakers are weaker than the Headunit in comparision so they do start to flap at 40 volume with base heavy music which made the sweap more neccesary than in other cars due to weak headunit but "ok" (semi-crap) speakers. Rear speakers don't really need changing since they are already quieter compared to fronts and never seem to recieve the same power even with front-rear-setting at neutral. I left mine stock thinking i'd have to push sound to the front on the fader menu but it's best in the middle due to the OEM headunits time alignments and varying preset volume levels.
Autoincrease volume when driving is off. These systems often involve modifying EQ curves as the volume automatically changes, we don't want that.
Focal speakers are always bright/shrill/powerful on 2khz and up compared to most other manufacturers. THey are perfect for Jazz and are high end, tuned so as well. One has to keep this in mind when choosing Focal in any car since it will affect perceived sound quality. If you like it bright, perfect! If you prefer a warm rich tone with crisp highs but no horn/birdsound/high freq electric guitar noises that overpower, look elsewhere OR get a DSP. Or Headunit with at least 12+ frequency adjustments.
Focals are made for a SQ-curve. It all makes sense when considering this (I've had Focal ESK100 in BMWs on stock pro logic amp, same sound imaging) BUT the GR Yaris has its tweeters very near the front window. This causes some extra treble acoustics which aren't always desired. Door-mounted tweeters are fine but glass reflects sound too well sometimes and causes it to resonate which isn't loved by all ears.
If you get the Focals, or any Focals for that matter, either go -1 treble +-0 midrange +3 base, or higher if you prefer more base, for a decent counter to the stock headunit curve but maintaing a SQ-curve. It'll make sense at speeds over 30mph. Our cars suffer from alot of tyre noise and roadnoise which basically kills most sounds sub 800 hz, or lowers them by ALOT when driving, hence having to overcompensate. When standing still even the stock speakers are OK up to volume 35, except not detailed nor bright, base is fine. Focals with sound proofing does play into the cabin at the same volume eventhough they're more sturdy heavier-coned speakers.
SQ curve that's desired for a car when wanting "good" percieved sound while moving: (It also requires solid doors to prevent resonance at 200-700hz)
This is the cars stock headunit curve as discovered by Magnus:
The dropoff in dB between 2khz and 6khz is where the Focal normally overpowers, makes it a quite natural fit in the end. Above 7khz things go bright though, hence why I recommend -1 on the stock equalizer if you run without external DSP/EQ. The naturally high mid is fine for Focals, doesen't sound echoy or boomy since it's a quality speaker even at this cheapskate pricepoint (200 bucks for a Focal kit IS cheap....Proper ones start at 400 pounds and up usually so it's no highend speaker per se, compared to regular aftermarket speakers for teenagers it is however.)
Edit: It seems like the stock EQ's Base and mid are much closer to eachother in terms of affecting oneanother compared to mid and treble. They also intertwine of course but not AS much. A setting of +2 treble -2 mid +3 base will therefor sound ok stock, awful with aftermarket. Alot of midbase will be lost due to the -2 mid setting making the curve drop off quite alot towards mid, which drags along most of the midbase and lowmids with it. Keep it in mind.
Lastly: I tried using the Spotify Equalizer to set up a curve similar as the one up top but I noticed something odd. For some reason when using the Spotify EQ, there'd be strange clipping sounds from time to time. Not due to quality of song or even overall volume. Disabling the Spotify EQ made it cleaner in regards to unwanted distortion in tweeters etc, instead chose to stick to cars own equalizer and it's fine. When driving

Bit base heavy and not entirely super bright when standing still.
I had the very same issue in my previous car, a 2018 Mustang GT. Quieter car due to insulation and materials, swapped every speaker in it and did the same insulation etc alongside everything in the rear half of the car due to a chunky Rockford sub. That ran several amps and a DSP but showed the same sound tuning issues, same tyres after all but much wider: Setting up a EQ curve while standing still means nothing on the move. Percieved sound is SO different, like I said earlier, that anything that sounds good in the parking lot will be very tinny and lack base/warmth on the move. The GR Yaris is just more of the same but WAY worse due to aluminium doors, carbon roof, less sound insulation ....so yeah! Sorry for the colossal post but i'm an audiophile who left the world of sweet music for good, settling for "meh it's ok" with this car, after the upgrade.
At least I can crank the volume and not fear for blowing speakers with the Focals! But the magic is in the sound deadening, insulation and the equalizer with this car. Less so the speakers.