I was quite curious to know how much of an improvement this would bring so I gave it a quick try. This is quite hacked, brought just far enough to measure the improvement (which is significant, with compilation taking on the order of seconds and an orders of magnitude reduction in compiler allocations). That being said, you might find it to be a useful start. My next step would probably be to provide a Data.Vector.Storable.Sized module which exports a type synonym,

type Vec = Data.Vector.Generic.Sized.Vec Data.Vector.Storable.Vector

and perhaps reexport the remainder of Data.Vector.Generic.Sized. You could also do the same for the remaining vector varieties.