I think you're missing a crucial point here: while duralumin boosts power very high, it still hits a limit. Feruchemy has no limit other than the amount of power you have available and are willing to spend. Pewter = strength X. Pewter+duralumin = strength 50X. Feruchemy with enough power = strength 10000X, though that much power is hard to get.
In the beginning of Hero of Ages, when Vin and Elend are fighting an Inquisitor in the midst of a Koloss army, there is one instant where the Inquisitor moves ridiculously fast, to the point where Vin is absolutely certain that not even duralumin could explain it. This Inquisitor has an extra spike that we, the readers, know grants the feruchemical ability of speed (I don't remember which metal that is). Combine that degree of power with speed, strength, and mental speed all at once, and you have something not even a duralumin-fueled mistborn can match, for as long as the power holds out. A feruchemist tapping enough speed could, in fact, move his own body faster than a duralumin-pushed coin.