Iv been playing guitar for awhile now and have recently been concentrating on speed techniques. iv got my picking hand as fast as I will ever need for fast controlled alternate picking and fast sweep picking, but am now concentrating on my left hand technique. My sweeping is really fast and my alternate picking is pretty fast as well but I am trying to push my speed towards MAB speed. I here alot of people saying to put my thumb directly in the back of the neck for the most speed and I do that for the E A and D strings but when it comes to the bottom 3 strings i really feel more comfortable with the top of my palm touching some of the neck and my thumb position more towards the top of the neck if not over it. My question is: will my thumb position for the bottom three strings hinder my ability to reach incredible speed in either sweeping or alternate picking? Also, stretching is not a problem in most situations and if it is I put my thumb slightly down. Thanks for any help in advance.
Different Strokes for Different Folks my friend.

I have seen John McLaughlin not even positioning his thumb on the neck at all, just letting it hang over, and lightning fast.

To have a control to this, Paco De Lucia, another guitarist in his [John McLaughlin] Flamenco Trio, And he is constantly putting his thumb behind his guitar.

What I personally do is always getting my thumb at least paired up with the middle finger of your fret hand, but when stretching i let it go as far left as I can make it, and also slightly down like you at first but I find it better practice to try not to, haha.

Hope this helps.
It seems me that what is properly taught is having the thumb positioned right around the center of the back of the neck.
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