Well yes and no :).
The 122 bhp has internals to take up to 300bhp (or close to that). So does the other engines.
In fact, some say that the 122 bhp internals, being a newer engine, are in some better than the twincharge ones (like the injectors for example).
You are right, 140bhp and 170bhp cars have identical engines. As far as I know even the exhaust is the same but with different tips.
About the brakes, in Germany the 170bhp cars have GTI front brakes and normal 256mm rear brakes. In Romania (and other countries as well), all 170bhp cars have 288mm brake such as the 122bhp or 140bhp cars (rear is similar 256mm).
I think that all 1.4 tsi engines are ok for tuning as long as they are not raced (trackdays are ok is cooling laps are done).
The twincharge has an additional issue to the single charge one: engine cooling.
The other problems are common: piston 4 lubrication, drive chain issues, intake camshaft pulley issues.