Most bows will lose 1fps for every 4 to 5 grains added to an arrow. I would not let speed determine my goal to increase FOC.
With a Broadhead I would not accept less than 12% FOC and prefer more. Things I do to get my foc up if need be are using heavier inserts, feather fletch, lighter nocks, no wraps, or as you mentioned go up to a heavier broadhead.
Those are just different things you can do. I don't necessarily do them all on every set of arrows.