According to xBanzai89 in the following link, this is true:
http://hardforum.com/showthread.php?t=1642982

According to the link that KTE provides that is in the following link, a benchmark was done of a Deneb versus a Thuban, but the Thuban had two cores disabled:
http://www.lostcircuits.com/fo...wtopic.php?f=3&t=4068

In the LostCircuits link, please, view pages thirteen and fourteen; the CPUs appear to have the same speeds with Turbo deactivated of 4GHz cores, 2.6GHz uncore/CPU-NB, and DDR3-1600F at RAS: 20 and CR: 1.

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Lad

I have had this name before the public announcement of L4D.

I rather doubt the IPC is different, as the Deneb-Thuban-Zosma are the same microarchitecture.
I shall speculate that thermal throttling would be the cause of Thuban giving a weaker score.

The Thuban had the higher benchmark result of the two on the last test that is on page thirteen; page fourteen has the Thuban with faster RAM tests.

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Lad

I have had this name before the public announcement of L4D.

The following linked page discusses Thuban's shared L3 cache possibly being the cause of worse single threaded performance versus Deneb, and it shows similar results of Deneb having higher I.P.C. than Thuban:
http://forums.anandtech.com/showthread.php?t=2078725

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Lad

I have had this name before the public announcement of L4D.