Do ALUs perform all/most N operations before running through the multiplexer?
Using any modern synthesis tool you can be reasonably sure it'll optimise away any redundancy it possibly can, to the point of sometimes eliminating the entire design if it wasn't tested properly in simulation first. 

So, yes this is inefficient; but synth will fix that. 

Better to keep what you write simple, clean and clear. 16 ops and a 16:1 MUX fills that bill.