The GX200 venturi is slightly bigger but I dont think you will get any noticeable gains in power. Strigoi hit the nail on the head with his post.
But if you have a GX200 carb laying around definetly bolt it on, the jet from your GX160 will probably be all it needs. Also a GX200 cam would give your GX160 a little boost too, oh and lastly a GX200 head gasket, it is thinner then the stock 160 head gasket so it ups your compression a little.
Now going the other way if you want to make your GX200 faster put your GX160 head on for more compression.