Hmm, it would appear that the Nehalem-EP has a different level of turbo to the desktop i9xx.

From the documents that came with the ES CPU's (X5560 @2.8GHz) I got, the Turbo specs are as follows.

Turbo active Cores 4c 3c 2c 1c
Max Turbo Frequency 3.06 3.06 3.20 3.20
Max Turbo Bin Upside 2 2 3 3

Now, these are different for the other CPU's in the range.

5502 has no Turbo
5520 and 5530 have Turbo Bin Upside of 1 (4c and 3c) and 2 (2c and 1c)
5550/60/70 Have Turbo Bin Upside of 2 and 3
5580 Has only 1 Turbo Bin Upside of 1 on a single core.

I'll try some more tests in the coming week to see what effects this has on the benchmark.

Cheers