Yeah, those are big, big heads. It has been suggested (shtr) that another category needs to be added: +65lbs KE for those really large jack knife style mechs.
I'm inclined now to put the slipcam style mechs in the 45+ category, since the ke required to open those is next to nil. You still have to deal with the larger cut, however.
Again, this is a general guideline we're trying to formulate. There will certain heads that won't match to the ke requirements. Case in point is the Rocket Steelhead. Should be in the 45+ category, but I think no should be afraid to use it if their setup generates less kinetic energy. That little mech is renowned for it's penetration ability.