You're still trying to defend a position "spiritualism isn't about a god" which I don't actually attacks. I'm not saying "spiritualists all about the god!" here. That's my opponents saying "hell, spiritualists are about gods and spirits, psi about gods and spirits, THAT'S why materialists can't be psionics!". I believe, spiritualism isn't religion (even if game actually moved that way), you can not to believe about gods and spirits, and you don't need to be religious man and blindly pleasing spirits from That Side to have psi.
I'm against another point. I'm saying that if materialist sees that some meditative practice actually allow you to read minds, they will follow this practice. Not because they are spiritualists, but because it allows to read minds. I'm saying that if technique gives you some benefits, materialists will follow this technique.
Please, look into "Mind over Matter" event. No, you haven't reach any enlightenment. It's just happened. Why? Nobody had any clue. MAYBE it happened because all empire SUDDENLY became spiritualists, but it's strange enough - no spiritualist sciencist in-game offered it as explanation. It's not "our thorough pursuit to spirituality ended at last".
Also, you know, psykers in WH40K aren't actually nesserely casting away material things. Quite the opposite - often psykers using their power to gain more "earth" power and material wealth. Also a long healthy life. Because you don't actually need "casting away material things" to use such an objectively exited phenomena as a Warp.
Just in case you don't know. In WH40K you can actually haven't any focus (and there is a doubt about soul) and use stable psy. That's psychic servitors of Adeptus Mechanicus, that grants AM separate "mail" network. Also there are a lot of artifacts that don't have any soul and still able to produce psy effects.
So no. They don't have to focus, even if it helps.
Because, once more, Warp existence is objective fact, not subjective.