Yep newt, as far as I know, a downdraft fuel delivery setup to the combustion chambers is supposed to be an optimal arrangement. However, I don't think in case of the Norton Commandos or for most of the older Japanese inline fours there was enough of a canted forward cylinder head design for the downdraft effect to supply much on an increase in power.
I think in most cases back then, it might have been a way of both lowering the center-of-gravity and also a means of putting more weight forward, which is preferable for better handling on high-performance motorcycles.
Btw, I saw in this month's Cycle World that BMW will soon be selling a new water-cooled R-series (Flat Twin "Boxer") motorcycle, and they've turned the cylinders 90 degrees forward, so instead of the fuel feeding from the back of each cylinder and the exhaust coming out of the front which all the Boxer twins always had, the new R's will have a downdraft fuel delivery to the top of the cylinders and the exhaust will exit from the bottom.
