What would happen ..... not a thing.
So long as the current regulation for fuel flow and materials remains the same, there won't be any increase in power output.
You could go from 1.6 L to 6.1 l but so long as you don't permit any additional fuel beyond the current limit of 100 kg/Hr at 10,500 rpm and up, no change in output.
Could quite well be that a 4 cylinder 1.6 L engine would be more efficient, but just try to sell that idea to Mercedes, Ferrari or Nissan. Good luck.
Personal motto... "Were it not for the bad.... I would have no luck at all."