I notice you, Mikepage, are from the great white north, so I would suggest using a different suspension setup for winter, so you have more ground clearance to clear the snow on the road, and more traction on the slicker surfaces. Since you already have the OEM FR-S front springs, I would suggest getting some stock BRZ rear springs, and mounting the FR-S springs in front, and the BRZ springs in back. The stock BRZ rear springs are softer, so they will hold your back tires to the ground better.

I'm using this setup right now, and it gives more traction to my stud-less snow tires, because the tires stay in contact with the ground better with the softer suspension. Stud-less snow tires have less rubber in the contact patch, so they are actually more prone to spin than my summer Michelin PSS2's.

How to get a better street/stiff feeling
 

You can just say Mike :) and I live in Quebec Canada.

I ride on Michelin x-ice xi3 since last year and it's fine with my current clearance. I don't want 2 different setup...

Do you watn a more firm ride, or a harsher ride?

Both fall under "stiff". Harsh is easy to do, firm will require a bit more money.

How to get a better street/stiff feeling
 

Are you able to give the solution for both then I will decide after that :) but maybe more firm anyway...

Why do you want stiffer? Do you find that your current spring rates aren't high enough for the grip you have? Do you have a specific issue you're trying to fix, or just want it to ride more like a race car?

Stiffer doesn't automatically add grip, same thing with removing body roll.

As for bouncing, that comes from poorly matched springs/shocks, typically spring rates too high for the valving in the shocks.

This.

If you want stiff without sacrificing grip, it's not going to be cheap. I'd budget at least $3000 for the coilovers, plus alignment components.

Mmm ok then maybe I will concentrate my budgets on alignment components only then I will got a better handling.

dollar per dollar, nothing beats suspension mods (driver mod omitted ofc). Was a mantra at nastysoc (nasioc ;) ) and holds through on this platform.

But as wisely said above, a goal should be in mind and wallet loose if you want a comfortable stiff setup

It seems @wparsons already beat me to it :)

What do you suggest?

Camber plate and camber bolts for the front and LCA on the rear?

Do you want it to handle better, or feel stiffer?

If you want it to handle better, you'll need to know what exactly you're trying to address. "Better" without knowing what you're trying to improve is impossible to help.

How to get a better street/stiff feeling
 

I think I will start with Tein Street Flex and make my setup from that :) I got a good deal on the forum right now. Not everyone will be agree with that but anyway... It's my car.

I really appreciate advices you gave me @wparsons @CSG Mike.

At the same time I will be able to adjust the camber on the front because they have camber plate.

What do you think?

You may want a rear LCA for best results (may not need it either) but that sounds like a change that may get you where you want at the moment and help you learn a bit too.