With VCDS, look at boost specified, boost actual, intake air mass g/s and actual torque. You can use intake air mass and torque to estimate BHP. Take the car for a pull in 3rd gear from 3k to 5.5-6k and you should get a good enough reading. Or if you can do the same in 4th gear, it's even better.
My cracked k03 turbo, on a stage 2 map, was underboosting by 0.3-0.4 bars throughout the powerband and making somewhere between 220-230hp