Yes, taking the hydromorphone and percs would definitely enchance one another, but if you plan on snorting the H-morphone(which you should

) then swallow the percs first, then wait 20-30 minutes to rail the H-morphone.
And yes, I meant 1.5 mg! I must have been real high to screw up basic math...
And yeah, oxycodone is also converted by the liver, but it is converted into oxymorphone(vs hydrocodone, which is converted into "hydro"-morphone).
But what separates the 2 is that hydrocodone is completely useless on it's own. Thus, ANY effects you get from hydrocodone are from the small portion that is converted into hydromorphone. This makes it a "pro-drug", because it is simply a vessel for it's metabolite, which is responsible for the effects.
Conversely, oxycodone is actually a potent opioid all by itself. ALso, only a tiny amount of oxycodone converts into oxymorphone. Thus, most of oxycodone's effects are actually from oxycodone itself; granted the conversion to oxymorphone provides additional effects, but it is small enough that even if none of it gets converted you will still get most of the effect from oxycodone. Yet if none of your hydrocodone gets converted, you won't feel a thing. That is why you CAN snort oxycodone, but cannot snort hydrocodone(that, and the fact that hydrocodone is always mixed with APAP)
Note that the conversion of the -codones into the -morphones is mediated by the CYP2D6 enyzme. SO people who are defficient in that enyzme will get little to no effect from hydrocodone, but still get full effects from oxycodone. ALso, oxycodone is MUCH more potent per mg than hydro. Yes, some converters say 1.5x the strength, but that is inaccurate. it is more like 2-3x the strength in most people(and let me stress, most people)