It is not 50% increase on your normal yielding speed, but it adds 50% in to your total yielding speed.  So if in your first example your yielding speed is 400%, with the 50% speed-up boost your yielding speed is not 600% but only 450%.  This means your yielding will only speed up 12.5% in this example, it will be different for you based on your normal yielding speed.

When you use your hero which should have a lot of yielding speed bonus then a speedup does not add much more, those speedups help most when your march doesn't have a hero or shaman in the march.  I usually use them when I have multiple marches out and especially when I am doing smaller size marches because I'm concerned about being attacked.

They also work great right after a global event is over where you can yield 10+ lv. 5/6 invader liars, especially overnight.