Increasing unit size in Iraq?

I've often been amazed at the ability of the Taleban in Afghanistan to repeatedly mount large unit operations, 100-300 militants, despite the 20-50% casualty rates these operations incur.

In Iraq, with a far greater US presence, more air coverage, and the complicating factor of the civil war, unit operations in Shia and mixed areas have generally been limited to 20-50 with an occasional mount by the Shia militias up to 100. So, when I read this out of Najaf today, it caught my attention.

For the past several weeks, Sunni insurgents, including Arab fighters from outside Iraq, have stockpiled weapons and dug trenches amid the orchards in apparent preparations to attack the thousands of Shiite Muslim travelers observing the religious holiday of Ashura, Iraqi officials said.

Iraqi police stormed the Zarqaa area early Sunday morning, but took heavy gunfire from the orchards, where an estimated 350 to 400 fighters were entrenched, according to Col. Majid Rashid of the Iraqi army in Najaf.

Now, I don't know if this is a one off operation by the Sunnis to strike Najaf at Ashoura, but the ability to stage that size operation over that period of time in Shia territory doesn't seem to indicate a situation coming under control.

On CNN they said the target of the effort of these Sunnis deep in Shia territory was to kill a number of influential Shia religious leaders.

I would guess that the Shia faithful are wondering today, "Where were the Mahdi?" and "Why is Maliki's government taking away our protection?"

(On the bright side, this Ashoura attack does seem to have been circumvented by government forces.)

Later: Iraqi officials say 250 killed, and it sounds like a oneoff. CNN also just reported that Sistani was the ultimate target, and, per Iraqi officials, that the insurgents were a collection of Sunnis and Shia(?).