For my RS, I am actually running an 02 forester transmission and with stock clutch master and have no issues with the pedal sticking. Since you replaced with a new master cylinder, make sure to bench bleed the brake master cylinder if you haven’t already done so to get all the air out. If not done, there is a big chance you still have air pockets in the master cylinder restricting fluid. Once it’s bench bled, you will need to bleed the clutch slave. Pedal will stick to the floor while bleeding but as you get all the air out it should be back to normal.