RE: Power needed to open an Expandable?
Almost any amount of power will open them, where the power comes into play is getting the penetration you need to reach the vitals or get a pass through.
I shot spit fires with a bow using a bit over 40 ft/lbs of KE and 350 grn arrows. They opened fine but lacked penetration. There are better mechanicals than the spitfires for lower KE set ups. I have a set of Nail Drivers that work very well.
Pushing a head through some material by hand in no way simulates shooting one at game. Mine opened on thin cardboard when shot from my bow.
Paul