The thing with Bullet Dance is that you're sacrificing the in-between time (and potentially ammo conservation) for extra speed, as unexpected as that may seem for pistols. Whereas a "normal" pistol strategy is to fire one (or two with DG) at a time and check your situation after each attack, allowing for a very flexible set of tactical opportunities, using MBD changes the strategy to something more akin to a rapid-fire build (or burst build as I've started to call it), plenty of shots at once but you're potentially vulnerable after each volley. In addition to this, pistols are very unsuited for burst-fire due to the extremely limited clip size...this can be easily remedied with a couple of speedloaders and some points in Reloader but it is certainly a nontrivial concern.
If we want to look at this from a quantitative perspective, however, we need to do some comparative analysis on pistols using fairly similar conditions.
Suppose we have the following traits: SoG(3), DG(1), SoB(2).
Pistols in both hands will deal (2d4+5)x2 = (7-15)x2 = 22 damage at a speed of 1 - (0.2*3)(1.2) = 0.48s, which comes out to 44.83 DPS, ignoring reloading and accuracy and other junk.
Now let's add MBD and TH(1) into the mix.
Pistols in both hands will deal (2d4+5)x4 = (7-15)x2 = 44 damage at a speed of 1 - (0.2*3)(1.2)(1.5) = 0.72s, which comes out to 61.11 DPS, or an increase of 16.28 (36.3%) in DPS.
Now let's up the ante to TH(2).
Pistols in both hands will deal (2d4+5)x6 = (7-15)x2 = 66 damage at a speed of 1 - (0.2*3)(1.2)(2.0) = 0.96s, which comes out to 68.75 DPS, or an increase of 7.64 (12.5%) in DPS.
Now, instead, suppose we have a chaingun or plasma rifle with Fin(2) and SoB(3), which is unfortunately as powered as they can get without extended max traits.
Chaingun will deal (1d6+3)x4 = (4-9)x4 = 26 damage at a speed of 1 - (0.15*2) = 0.70s, which comes out to 37.14 DPS, ignoring other junk.
Plasma rifle will deal (1d7+3)x6 = (4-10)x6 = 42 damage at a speed of 1 - (0.15*2) = 0.70s, which comes out to 60 DPS.
Let's skip the TH(1) case and go straight to TH(2).
Chaingun will deal (1d6+3)x4 = (4-9)x6 = 38 damage at a speed of 1 - (0.15*2) = 0.70s, which comes out to 54.29 DPS, or an increase of 17.15 (46.2%) in DPS.
Plasma rifle will deal (1d7+3)x6 = (4-10)x8 = 56 damage at a speed of 1 - (0.15*2) = 0.70s, which comes out to 80 DPS, or an increase of 20 (33.3%) in DPS.
Now there are PLENTY of other variables to look at, but as a matter of damage alone, chaingun and plasma rifle keep up pretty well with MBD and have an extra trait to boot (which can be put into EE to even out accuracy concerns). Naturally if we're looking at the LONG term, SoG(5) beats practically everything, but this isn't until the truly late-game: if we're trying to find a build that can keep the player alive, we need to think of the traits as they're being collected. Anyway, this is just digressing on my part.
Overall, I'd say TH(1) on MBD is worth it, but not TH(2) unless you're well on your way to reaching extended maxes. Frankly I'm a little worried it's not quite good enough in a typical game, but is certainly useful by the end.
EDIT: This would be silly if I didn't even try to answer the original question...
How do Bullet Dance and SoaG 5 interact? 6 shots in .3 seconds? less? If I'm understanding things right you take .4 * 1.2 * 2 = .96 seconds to fire 6 bullets with SoaG 3 and normal/speedloader pistols. Are the times all added together before rounding and applying minimum delay?
I'm pretty sure the minimum time limit only comes into play at the end of everything, although I'm also fairly certain that the SoG(5)'s bonus is capped at -90% firing time anyway. This would lead to calculating 0.10 * 1.2 * 2 = 0.24s for TH(2) MBD attacks, quite an impressive speed when you consider six shots per attack. On the other hand, it might also have been reduced to, say, 0.01 firing time, meaning any multiplers are mostly ignored and the minimum time limit kicks in so that even TH(2) MBD attacks are 0.10s long. (Methinks we need to change SoG(4/5) a bit.)
Similarly, how do you keep this beast fed? You spew 10mm at a ridiculous rate and waste tons of ammo overkilling many enemies with your crazy 66 damage bursts before the trait cap. I guess the high ideal is dual blasters, but I don't see that happening very often outside of Ao100.
Keeping track of ammo is about the same as keeping track of ammo with, say, a gatling gun, which also shoots six bullets at once: you kill as efficiently as possible. Stack enemies on top of each other, switch to other weapons if you can afford to. With pistols, however, you can always switch to a single pistol and only shoot 2/3 bullets at a time, in case ammo is low.