It looks like the problem has been solved finally!
The mechanic changed the TP sensor and I brought the car home. The next day, I took apart the TB for a thorough cleaning (it was very
dirty and the butterfly was a bit sticky). I also changed the TB gasket. Where the mechanic thought was the vacuum leak (where the boost control solenoid meets supercharger), I stuffed an O-ring from outside just to be safe.
Now the car is running better than before. Good idle, good pickup, good boost. Even the shudder problem that I previously thought was related to the transmission is gone completely!
I don't know if it was the dirty TB (I would think so), but I certainly would be cleaning it every 50,000 km from now on.