Yeah, it's pretty stupid. I'm hoping they adjust it eventually, but won't hold my breath.
Anyway, as promised I ran some quick numbers. I used a 28 damage enspell, which is a high-end but not top-tier build (lacks the new enhancing magic skill body from Voidwatch) plus capped skill + merits. Also used 8/8 merited sword and dagger since we're comparing Mandau.
Started off with a comparison inside Abyssea. As I expected, STR Shamshir +2 came out slightly ahead of DA Khanda +2. More interestingly, Mandau 90 performed best out of all the weapons I compared even with just base damage/delay. Oneiros Knife was the closest contender inside Abyssea assuming critdmg+5%; if it's a 10% bonus then it would edge out Mandau. Chimeric Fleuret was next, followed by Sagasinger, then STR Shamshir +2 and DA Khanda +2. Chimeric Fleuret and Shamshir +2 would gain some ground vs higher level enemies.
Outside Abyssea, the order gets shuffled around a bit depending on what you're fighting. For new VT+ content expect Chimeric Fleuret to take the lead, followed by Sagasinger, Mandau, STR Shamshir +2, and then DA Khanda +2 and Oneiros Knife. For older content where defense is lower and thus physical damage is a greater part of your DPS, Sagasinger and Mandau are almost identical with Chimeric Fleuret close behind, followed by STR Shamshir +2, DA Khanda +2, and Oneiros Knife once again.
If enspell resists are an issue, Sagasinger is probably your best bet. Chimeric Fleuret and Mandau are still strong choices due to their strong damage/delay and the fleuret's DA+4%, though STR Shamshir will perform comparably. If you're /DNC instead of /NIN, the low delay weapons are slightly less attractive. Chimeric Fleuret gains ground if you need the accuracy+5, though there's also DEX Shamshir +2 to consider at that point.