I have some Hammerhead Trads. They are comparable to a 55/75 spine. I just barely got them to bareshaft out of my 64# RER. I have a 175gr head and insert bushings up front with the standard aluminum insert. The back of the broadhead is right at my finger when I reach full draw. It may be due to the taper, or how hard an RER shoots, but you might need to get a slightly heavier spine than you think or buy the test kit.
They fly great. I really like them. As far a penetration goes, I can measure how much arrow is in the dirt on the back side next time I shoot a deer, if you would like...
God Bless,
Nathan