Hi landru,
This post reminded me of my second piano teacher from 6 years ago, which was the first teacher to point out my thumb position! (which I never really noticed by the way) Yes the thumb should be pointing inwards instead of outwards, but for some people it's just not that way. When I play scales, my thumb is curved outward, but I have absolutely no problem playing fast, and speed happens to be my strongest characteristic.
My first teacher was very big on scales and exercises, and this is what helped me with speed, so this is what I would suggest. Also, since I was good in playing fast, my first piano teacher assigned many fast pieces, and this is probably why I hate slow pieces to this day...ha! You just have to get use to playing with your thumb that way, and eventually your thumb will learn. It's just like learning a new technique....first it's very hard, but with practice, you hands adjust.
- Pet