Exactly. If you are already at hit cap with
[Illumination], replacing the staff with
[Barb of Tarasque] and either
[Mystifying Charm] or
[Talisman of Heedless Sins] (your only sensible choices) will be a downgrade to your DPS (according to Rawr's model anyway) unless you can shift other items of your equipment to bring that +Hit back into play.
And that's the problematic part: at this stage of the game, your +Hit comes from either T9 items which you have to endure in order to get your set boni or from items that have no spirit but offer a rating combination of haste/hit or crit/hit like
[Shroud of Displacement] or
[Belt of Biting Cold]. The ideal item has crit and haste without spirit such as
[Leggings of the Awakening].
Or to explain it differently:
You can categorize all caster-items into three categories of quality by their combination of spirit, hit, crit and haste (assuming they all have stamina, int and spell power):
1st grade: crit+haste
2nd grade: crit+hit, haste+hit
3rd grade: spirit+hit, spirit+crit, spirit+haste
What we strive for in our equipment ist to have as much items of the 1st grade as possible (unfortunately they are not available for every slot) with only as much 2nd grade items as we need to achieve our hit and haste caps. 2nd grade items are only stronger than 1st grade items as long as they fit into our hit cap. 3rd grade items are ALWAYS the least desirable choice if no item without spirit is available for that slot.
Blade of Tarasque is 2nd grade and the available off hand items for it are 2nd and 3rd grade. Illumination however is a 1st grade item and that alone makes it better despite being 13 item levels lower (which is also partially compensated by two more sockets and a stronger enchant).