Well, have to look through a few things to get an answer here.
First, without Impetus, I'm not seeing the Dread Jupon as significantly ahead.
Vs DC Dynamis, Dread Jupon at 6306, Qaaxo A at 6292
Vs Delve Fodder, Dread Jupon at 5488, Qaaxo A at 5512
Vs Kamihr Raaz, Dread Jupon at 4639, Qaaxo A at 4663
Basically, as the mobs get tougher, Qaaxo is getting further ahead.
Why is this? Well, looking at the item stats (and factoring out acc/att conversions), we have Qaaxo with 9 att and 2% TA, vs 8 str, 13 dex and 19 acc.
Vs low mobs, the acc is irrelevant, dDex is probably capped, and att might be close to capped. Thus it becomes a showdown between 2% TA and the WSC of 8 Str. From the numbers above, WSC slightly wins.
As att comes uncapped, the edge shifts slightly towards Qaaxo. Looks like 24 points of damage in both cases.
Then you move to Tojil, and accuracy comes uncapped in the unbuffed setup, so suddenly the Dread Jupon leaps ahead: 1929 to 1746. dDex is also significantly under cap, so that becomes significant too.
Given that for the weak mobs, it doesn't really matter which one you use, we'll focus on the Tojil scenario since that's where the difference becomes interesting. We'll also make sure we have a Madrigal going, for accuracy, else we'll have to completely reconfigure the gear set for an acc build.
If we look at the Data page, we find that using the bog-standard equip setup with no consideration given to accuracy, the Dread Jupon puts us just above the acc cap, at 96.5%, while the Qaaxo A is at 86.5% -- 10% hit rate difference, or 20 accuracy (of which only 17 was strictly necessary). At that point we have 2086 damage from the Qaaxo harness and 2252 from the Dread Jupon.
So, assuming Impetus was active, how many sequential hits would be necessary to match that amount of damage while using Tantra Cyclas +2? And at what point would you really consider Tantra+2 the superior choice? +50 damage? +100 damage? I'm going to go with a roughly 1% DPS improvement (which is in the 2-3 DPS range for this).
Tantra+2 @ 5 hits = 2197
Qaaxo @ 5 hits = 2195
Tantra+2 @ 6 hits = 268.101 DPS
Qaaxo @ 6 hits = 266.027 DPS
+0.78% DPS
Tantra+2 @ 11 hits = 2500
Dread @ 11 hits = 2509
Tantra+2 @ 12 hits = 2541
Dread @ 12 hits = 2533
Tantra+2 @ 14 hits = 290.919 DPS
Dread @ 14 hits = 288.208 DPS
+0.94% DPS
So in an accuracy starved situation, you'd want a good 14 hits in a row before switching from the Dread Jupon to Tantra Cyclas +2, although just 6 hits is still sufficient to start beating the Qaaxo Harness.
Of course even that's not the entire story. The Tantra body only needed 10 hits to cap accuracy, and during that entire time it was gaining crit damage as well. Given that Qaaxo and Dread were so close on lower level mobs, why do we need to add more hits beyond that in order for the Tantra to beat the Dread, even though it would have already beaten the Qaaxo?
Well, that starts digging into the dDex crit rate. In this particular case, the Dread Jupon was at a +3% crit rate from its 26 dDex, relative to the baseline 5% crit rate (which is all that Tantra gets). Qaaxo only had enough to put it at +1% crit rate, so that's a notable advantage the Dread has over the others. What would have happened if the whm had given Boost-Dex?
Well, Tantra+2 goes into the positive with +2% crit rate. Qaaxo is up to +4% (so 2% ahead of Tantra+2 instead of the 1% it had been at). And Dread goes up to the +15% cap, gaining a massive edge against the other two.
How massive? Well, at the 14 hit mark where the Tantra body started to pass it before, it's now 2682 for the Tantra to 2836 for the Dread. You need to go up to 21 hits in a row before the Tantra even ties, now, and 23 hits for it to be 1% ahead in DPS.
So what conclusions can be drawn?
If you're giving any consideration to accuracy, expect to want a fairly large margin of hits before you even consider switching from Dread to Tantra. Given that you probably aren't going to hit the dDex sweet spot as cleanly as this example did, I'd probably aim for over 20 consecutive hits before switching bodies.
If accuracy is not a problem, you still have dDex to account for. It's fairly easy to cap even up to the Kamihr Raaz, but once you hit targets where you seriously need buffs (eg: Serac Rabbit level mobs, delve bosses, etc), you probably need to consider it. Of course those are the same mobs where accuracy is going to be an issue, so you can probably treat it as going hand-in-hand.
Thus, if acc matters, dDex matters, and if acc doesn't matter, dDex is likely capped. In a capped situation, Tantra ties with Dread at about the same point as it ties with Qaaxo (~5-6 hits). In an uncapped situation, both Qaaxo and Dread have a chance to gain a fair bit from dDex, but Dread has a much higher chance, and I'd consider the odds of it for Qaaxo at a slightly lower weight.
I would thus probably define target points of:
No acc needed: Tantra at >6 hits
Acc needed, Qaaxo: Tantra at >10 hits
Acc needed, Dread: Tantra at >20 hits