The swaybars themselves?
The 2022 GR86 uses the 2017-2020 set up, 18mm front and 15mm rear.
The 2022 BRZ uses a new 18.3mm hollow front bar and a 14mm rear.
The equations for figuring out spring rate from swaybar dimensions aren't too hard and there are numbers posted on this forum somewhere. Have to remember the motion ratio for the swaybars as well (which can be different than the main spring motion ratios).
That new hollow BRZ front bar has been quoted as a mild 2.3% stiffer than the standard 18mm solid front bar. That's not very much but it is probably a little lighter which is nice. As with the aluminum front hubs you can see Subaru made effort (at extra cost) to reduce unsprung weight. Which is good.
With both springs and swaybars you have more front roll resistance and less rear roll resistance with the BRZ vs the GR86.
The rear swaybar mounting method is a related but different question. Like others said the new BRZ takes the subframe and subframe bushings out of the equation, meaning it acts without compliance from the subframe bushings. The subframe on standard bushings moves more than you think. Yes you can add subframe bushing inserts but something like a solid bushing insert or full replacement adds noise and is not great on a daily driver.
Does that make the 14mm BRZ rear act like a stiffer rear bar? Not really, but it's a more linear and direct action. Which is good IMO.
All the discussion about the playful nature of the GR86 vs the stability of a BRZ is largely the spring and swaybar rates, not the front hubs or rear swaybar mounts.
This is just my opinion but the GR86 using the old style front hubs and the rear swaybar mounts was simply cost savings. Arguably the solid vs hollow front bar too (2.3% difference in stiffness isn't much).
So why is the GR86 sometimes faster around a track? Well the new BRZ components are better but not a huge change. Meanwhile, off the factory floor these cars desperately need more front camber. Without that, more rear roll stiffness from the GR86 helps a lot to get the car to turn.
With the aluminum front hubs and new rear swaybar mount, the GR86 would be largely similar and still have the "playful" character.
The best handling 2022 would be a BRZ with GR86 rear springs and the OEM crash bolts for a tiny bit more camber. Maybe the GR86 rear 15mm bar too.
- Andrew