That wouldn't work out too well. One of two things would happen. Either:
Originally Posted by JC8
A) Only the players would get Instagib Rifle, making the game too easy (every Strogg including the bosses would get killed in one hit.), or
B) The Stroggs would have Instagib Rifles as well, making the mode WAY too difficult to sit through.
To be honest, mutators like Instagib and Buckshot might work brilliantly with competitive multiplayer gametypes, but in co-op modes like Invasion, they tend to ruin gameplay for complicated reasons.