Swedish speakers in Finland are not immigrants anymore than Dutch speakers in Belgium are, though (the Dutch speaking area also generally does better than the French speaking areas, for a variety of reasons).

it is interesting to note that since the industrial revolution it was the french speaking part of belgium that flourished due to its steel industry, and the dutch speaking part was the poor one. only when the steel industry stopped being competitive the economy switched away from industry to service where the french part stagnated and the dutch part started to flourish,

so clearly who does better has nothing to do with the language but with the economy and who has the better resources and is more adaptable to serve the current needs.