From what I can see, a HQ crafted piece has an equivalent stat budget to the same Darklight piece (e.g. HQ Darksteel Hands has same stat allocation as DL Hands), with the added benefit of being able to meld materia to it (potentially 5 times).
Now, the piece will be fully allotted to your attributes (i.e. as a MNK, you won't get any value from attaching +STR materia to a piece), but you can significantly tweak your secondary stat budget. Skill Speed, Accuracy, Crit Rating & Determination can all be added, depending on the native stats of the piece (you won't be able to add Acc to a piece already with Acc, but you would be able to add Det to it).
You're mostly correct.
Several pieces, despite having an existing stat, can still have their native stat boosted further. The HQ Darksteel Haubergeon, for example, has +19 Skill Speed to begin with, but can still get another +8 more from materia. Of course given that the DS Haubergeon lacks Determination in favor of Critical Hit Rate (the DL has Determination), I'd personally recommend compensating for that loss of Determination with some Savage Might materia before boosting existing stats further or adding something else (like Accuracy, unless you're really lacking in advance of the Dark Coils).
Same with the Heavy Darksteel Armor (combined head + body piece). It has +30 parry already but as much as +12 more can be added to it.
On that note, more people need to consider touching the DS/Vanya/Hippo gear with more than just a wary poke. It's true that not everyone knows a crafter who can pimp them out in it, but if you know a 50 Armorer, or a 50 Leatherworker, or a 50 Weaver, definitely see if it might be better for you overall since you'll be in Darklight for a while, given the 18 weeks worth of capped Mythology requirement for full relic (21 weeks if you count the +1 relic weapon). Some pieces are not worth the time and effort, true (I'm looking at you, Vanya Trousers, with your 3,400 tomestones worth of Potash/Coke/Animal Fat), but others are well worth either a few more runs over their counterpart (DS Haubergeon vs. DL Cuirass), and some even cost less
due to a fluke in Tomestone prices (1125 Tomestones/12 runs for 9 Coke for the Heavy DS Armor versus 14 runs for the Heavy DL Armor). In fact, I'd throttle any Paladin who did have access to a 50ARM but got the DL body/head anyway since the DS has Determination, a much more important stat, versus Critical Hit Rate. (I wouldn't really throttle them; just saying.) Edited, Oct 23rd 2013 2:27pm by Satisiun