Myvi? Handling? Pfffffffftt...
That car was never designed for handling. No matter how many bars you put or what adjustables you use, it'll never be good enough.
The Kayaba adjustables I've actually sat in a Myvi with this suspension and I found it to be quite decent in comfort and quite respectable for handling. But it's still not going to win GRA contests or out corner other cars.
How about you talk about what it is you really want to do with this car? Like out corner SUVs in corners? Or just be that little bit faster in corners? Or reduce squatting and nose diving when you accelerate and brake? Or just personal enjoyment? If it is the last option - then whatever you buy will suite you. Everything is better than stock in terms of handling.
GRAs have seen a lot of overturned Myvis because of overzealous turning/cornering. Ever since then, people have stopped using the Myvi for GRAs unless they are just there for the fun of it.
I've only ever seen one serious Myvi on Sepang with a Toda 4AGE engine in it with full race chassis and all.
Anyway, I'm digressing - my suggestion to you is buy whatever you think is good that suites your budget. Because ultimately it's going to make you happy since it'll be better than stock but be prepared to lose comfort.
Or are you looking to stock up on all the Ultra Racing bars? If so, you can most definitely try. That might suite your requirements too.
Anyway, happy shopping.