One of the best mods you can do on a GR86 / 2nd gen BRZ, delivered promptly and in excellent condition by GKTech. You get two springs to choose from, so pick your poison and prepare for a bit of yoga if you are installing this in a RHD GR86.
No exaggeration, the install will have you contorting yourself into the footwell, practically turning into a human bridge just to get your head in position to remove the clutch assembly (required on a RHD). I used needle-nose pliers, a hammer, a small flathead screwdriver and some self supplied grease. All up, it should not take more than about two hours.
I went with the 66% spring, and I can confidently say it was 100% worth the sore back the next day. It completely removes the kick at the bite point, making the clutch feel far more linear and controllable. The slightly heavier pedal is also not nearly as bad as the description made it seem.
After installation, there is a small amount of slack right at the top of the pedal travel now, but honestly, you will not notice it while driving unless you are really looking for it.
Overall I can without a doubt recommend the 66% spring - it's cheap and makes the car way more enjoyable. Haven't tried the 33% spring and can't say I see myself going through that install again to do so.