Would it be possible to add a check-box similar to to "Sync Fleet" called "Sync Salvo Speed" that when checked forces all missiles fired from the same ship and/or fleet to fly at the speed of the slowest missile?
For example, you may wish to launch a mix of normal and laser-warhead missiles that are otherwise identical, but they're going to fly at different speeds unless you add fuel to the normal-warhead one to make up the difference, which is a waste of fuel.
Alternately, could we have a design feature in the missile design section called "ballast" that does nothing but add mass/size, and costs nothing, to make matching missile speeds easier?