interesting. i think speed is significantly undervalued for guards, relative to per, bh, pass.
i also tried to do this myself, and what i ended up realizing is, it just didn't work. it can be a decent approximation, but to really use stats to rate players, i had to dig deeper. for a while, i spent a lot of time tweaking my ranking forumlas to correctly rank incoming players... i found i had to use cross products, like spd*per in a sg, to avoid over-rating players with say high per and low spd, which isn't very useful from an offensive standpoint.
also, eventually i got frustrated trying to calibrate a whole player at a time, so i broke it down. for guards, i used offense, defense, running the point so to speak, and other. for bigs, offense, defense, rebounding, and other.
finally, instead of 5 sets of numbers by position, i used maybe 10 sets of numbers by type of player. in the numbers in this thread, for example, the pg ratings will poorly rank a pg who has poor per but is otherwise incredible. it will put him below a lot of inferior pgs with better per. so, i made ratings on a type basis, instead of position. i wasn't even sure where to start at SF without first categorizing them as a particular type of sf :)