when i upgraded mine i saw a rough 30bhp gain and 40lb/ft torque increase.
Boost on a Stage 2 is around 22psi peak and holds 18 psi, stage 1 is around 16psi peak and hold at 12ish psi. but this can depend on the car as some turbo's are in better condition to others, along with the actuator. I found that i was maxing the MAF out and needed to go bigger maf to get more power after the upgrade.
And yes it is deffo worth upgrading from a K03 to K03s, it will put a smile on your face and make the car feel lods better. To get the most from it make sure you have all the supporting mods like, Larger TIP, Forge 007p, Cat back exhaust, Enlarged downpipe with sports cat or de-cat ( This makes a difference as it lets the turbo spool up quicker. Then get the turbo fitted and mapped. optional mod which would help produce more power is FMIC