With only 156K on the clock, you have a right to be skeptical.

Ask the mechanic why he wants to change the pistons. Report back on exactly what he says. Write it down so you get it perfect.

There are good reasons to change pistons, and there are good reasons to keep them. I have not priced them lately for the turbo engine, but, with the exchange rate the way it is, I would not be surprised if a set runs close to $750.00

He must provide specific info on what is not within spec. This usually is related to the ring grooves.

Report back with what he says.