Two thoughts here:
Have you tried adjusting the kilroy? On all of my lights I always play with the angle if the kilroy because I like to have the high and low very close to each other, some folks pull the kilroy up a bit to have more of a separation between levels.
Thought two, is your battery spring in the piston stock? If so this may be why the head gets harder to turn as it gets closer to the body. The springs in the new Ti lights sit up a little higher than in previous lights, putting more resistance on the threads on the head. Take the battery out of the head and see how much smoother the head twists all of the way down. With no resistance to the head it turns much smoother. Try cutting your spring down a bit and see how much smoother it turns and how much easier the piston is to operate.
I may be totally off the mark with these thoughts, but I thought I'd at least throw them out there for you...:shrug: