I think this is going to depend on your role in a raid setting. If your going MT, I would imagine the highest priority is having a Parry stat as high as it can be, since Dark Knight's actually get rewarded in different ways for Parrying. You'll want Reprisal up as often as you can and procs on Low Blow are nice also.
As for off-tank/DPS, I just can't with good merit advise spec'ing into Skill Speed. If you don't need to be in Grit, then you won't be and will be able to use Blood Weapon. Even with low SS this becomes problematic very, very quickly. You will drain TP like a sieve and FAST. Pushing that value any further aside from organically raising off gear upgrades just seems like it would be bad for long term fights. Maybe shorter bursty fights would be advantageous for some SS, but I can't imagine it would be very viable for most endgame, when we have been conditioned with long lasting fights with little to no downtime for recovery. I realize that DoTs would be stronger and stuff, and Scourge/Salted Earth would be affected positively, but I don't see that being a good enough reason unless there is some hard data to prove me wrong. When DRK runs out of TP, we have no choice but to spam Unmend, hope for Unleashed procs and continue until we have regen enough to attack normally. You lose a ton of DPS by doing this. I'm just not sure if that loss in DPS is covered by the gain in DoT ticks.
I'm also pretty sure no one has done a substantial amount of crunching enough to know how much worse off Det is now as well, so currently (imo anyway) the only value that makes sense raising via materia is Crit because we know the stat itself has been affected in a mostly positive way: higher value = more damage possible from the attack.
I'm not doing testing like you though, I had some STR 4's laying around and figured I would help more on the DPS checks in Bismarck EX if I just put them on the gear I still had that could take them. I haven't added any Crit to anything I'm using on DRK, I just noticed that my Crit value was naturally pretty high with w/e I was wearing at the time. Also the variations on the crit values being much more spread out then in the 2.x lifetime.