QUOTE(zeng @ Apr 18 2021, 05:01 PM)
I know your Yaris is new, but for some reasons you want 'handling' improved.
Depending on what aspects of handling you want improvement, imo you should start with
redo-ing another wheel alignment re-adjusting/altering what comes from factory, which you desire improvment to suit your 'taste'.
Briefly, for faster cornering/turning thrill you want to make 'as is' camber angles more negative.
Besides you can also ask alignment shop to adjust the front toes to be more positive or more negative for two different types of 'handling' you want it 'corrected' .
Typically front caster angles is not adjustable , but it appears to be not quite relevant with your 'problem' .
Rear alignment angles adjustment? Not sure of your rear suspension type and I suspect it is likely to be of no concern imo.
Spending $$$ on hardware currently may likely not achieving/resolving what you desire specifically.
Note: In case you are re-doing another wheel alignment, kindly take a screenshot (or ask for a print out prior to carrying out the job) and share here what is the specification range of Yaris alignment angles according to computer software.
Rear is torsion beam, nothing to adjust.
Front alignment, I don't think it is safe to do toe-out for street car. The front will be overly twitchy. Generally a little bit of toe-in is good for highway.
Floaty on high speed is common on the urban city car like Vios/City/Myvi/Axia, new Ativa. They all floaty at over 100km/h, they are design for city best 40-60km/h, efficient, light, easy to maneuver with tight turning radius, in traffic jam. Some 80km/h on inter satellite city is fine. Highway should be a couple of time a year, balik kampung use only.
There are very little things can be done without resort to full project car situation. Biggest problem is aerodynamic lift. Any road car has lift, spoiler merely reduce the lift, overall net force still upward (no or negative downforce), but bigger car has more weight and wider body thus less noticeable. Most effective is reducing the ride high, couple cm makes noticeable difference, so lower spring/coil-over helps (no need to be stiffer).
Other suspension mods are useless in the high speed straight line at regular highway. Unless you talking about Nurburgring, talking curb stone at 200km/h, then yes, but even that the mod is longer stroke and softer suspension setup, not firmer. Torsion beam, the beam structure itself is acting as ARB (that's why it is not an independent suspension). Adding rear ARB actually make the rear to slide at lower speed, remember feeling good (less body roll) is not equal to more grip. ARB is a tuning tool to balance front/rear grip (stiffer rear more over steer, stiffer front more understeer). Upgrade to stiffer rear need to come with stiffer front to maintain the balance.
Even if one determined to remove factory spoiler, replace it with a proper calculated wing (not ah beng ricer decoration). Let's assume 300kg of downforce, that's 4 adults weight. It is enough to change the pitch angle of the car, front nose will point 2-5 degrees up. (Remember modern car with HID/LED needs headlight adjustment also need compensate passengers load that influence to pitch angle) Then, in turn front become floaty, have to add front splitter to balance the car back. Bla bla bla... become fix A, break B, then C, a lot of effort + money, in the end overall drag increase, bad fuel economy as well.
In conclusion, my advice is always want/need a fast car, then buy a fast car. Frequent highway use at least C-segment. No budget either get used or delay gratification (live with what one have, then work harder for a couple more years). B-segment is best for city use.