Just guess.  Difference is marginal either way.

There is a big difference between the damage Shiradi does when used with spells and metamagic feats active or when used with ranged or thrown weapons. As an endgame weapon user the damage bonus of LD should do more damage than the damage from Shiradi procs. And you should be able to keep the stacks maxed with manyshot/10kstars/fusilade/doubleshot anyway. Shiradi is just for getting primal past lives or when you need the CC from nerve venom/pin/whistle. If Shiradi does more damage than LD your weapon damage + attribute bonus before melee power is gimped (even as a shuricannon with 1d2 weapon die).

For casters with metas, good spellpower and multi-proc spells on the other side Shiradi can be nice because it is a good damage option without a DC and has built in CC. If you can go toe to toe with the mobs and have a good enough DC DI does more damage than Shiradi imho, but if mobs tend to save against you elemental burst you are better off spamming MM in Shiradi.

In this case I am running a thrower LD and all trash fall to fast to care about, but I want to up my dps vs bosses so looking at twists for this purpose.  Most twists whistler etc only really assist with trash.  So now looking at twisting in prism/double rainbow or Echoes of the Ancestors: Primal for +1 dex and 10% attack.  But without numbers on rainbow it is a tough call.  May do the math later, just hoped someone had already done it to save me the effort.