I assume you are talking about AEG's here.
It depends on what you are calling lower quality gun. If it's something like $50 wallmart gun than don't even bother. If it's something like DBoys or Cyma, you want to redo all internals to your own liking and can afford the parts than why not? Those things already come with metal externals for the most part. IMO it'll perform as good as higher end guns. Keep in mind, external look of lower end guns might not be as good/detailed as more expensive ones, same goes for quality of material used.