It‘s only a temporary boost. Your Pokémon will always stay at 100 or less. If your Pokémon is level 90, you can get a temporary +50 boost - depending on the donuts you make - to boost to level 140. but your Pokémons real Level will not change.

(Looks at gen1 level 256 MissingNo)

This not-quite Lumiose City is home to newly discovered Mega-Evolved Pokémon and other Pokémon not usually found in Lumiose, some of which can exceed the normal limit of Lv. 100 with hyperspace levels.

The way they phrase it sounds like Planned Obsolescence to me. “Catch a new team of DLC Pokemon because your beloved old ones won’t be able to participate in the new feature!”