Hyper-threading and Primegrid LLR

I had my new i7-3770 on an Asus P8Z77M-Pro motherboard set up and running for the Primegrid SR5 challenge ... and then noticed something rather sad. My Pentium G850 was completing individual work-units quicker and overall my Core2Quad Q8200 was doing more work per hour. The Xeon E31220 (with a similar benchmark) was doing around 3 times the amount of work per hour.

I started disabling all the 'smart-step' settings in the BIOS and even re-flashed to the latest version. The problem persisted, so I checked the OS and couldn't find any scaling. The CPU temperature showed around 58'C and I couldn’t find any stray processes that were hogging the CPU.
As a last resort, I disabled Hyper-threading in the BIOS and suddenly CPU temps kick up to around 65'C and it burns through a work-unit about 10% quicker than the Xeon.
At this stage, I am assuming that the CPU knocks its own frequency down when all the threads are being used?? Whatever the cause, it is going to be a 4-core machine from now on!
I finished in position 199 for this challenge, which isn't too bad

Intel i7

Due to reliability problems with my office computer, I have upgraded to an Intel i7 -3770 @3.4Ghz with 16Gig ram and an SSD. It is running 64-bit Ubuntu 12.04 LTS and at the moment is running with Hyper-threading turned off.
This is going to hopefully speed up the rate at which I can find primes for PrimeGrid.

On the topic of PrimeGrid, as I write this, we are in the middle of their base 5 Riesel "Fathers Day" challenge and I dragged a few of the older machines out of the store-room for the occasion. The obvious down side is that these machines take well over 6hrs to complete a work unit.