I suppose if that's from the move thread then it has been thuroughly tested as to being correct?Well, since the max. Speed Ditto can attain with +Nature and 252 EVs is 214, then yes, Quick Powder would make that 428.
As for Ditto's Transform, here's what we've got in the D/P Analyses Move Descriptions for Transform:
[NAME]
Transform
[SHORT DESC]
Transforms into the foe.
[LONG DESC]
Transforms into the target, copying its stats (except HP), moves (at 5 PP each), type, and ability. The stats are copied exactly, so EVs, IVs, and level are irrelevant. IVs of the target are copied for purposes of Hidden Power, as well. Metal Powder and Speed Powder's boosts are lost upon Transforming. Choice Band, Choice Scarf, and Choice Specs keep their boost upon Transforming.
Does that help?
And is that saying that even the foe's hidden power type/power is copied?