Hyperthreading hurts performance
I found a interesting comment to hyperthreading on a Microsoft-Blog (know your enemy)
"It's ironic," said Ibbotson. "Intel had sold hyperthreading as something that gave performance gains to heavily threaded software. SQL Server is very thread-intensive, but it suffers. In fact, I've never seen performance improvement on server software with hyperthreading enabled. We recommend customers disable it when running Citrix and our software on the same server."
It seems chip multithreading is harder to implement than everybody thought. But remember, at the end yperthreading is like power management on x86-processors: management of inefficiency. It was invented to rise the utilization of the netburst archictecture as several function units had nothing to do most of the time (As far as i remember the Power-processor had a similar problem at least in in their fourth incarnation)
You should try Solaris on Niagara: An operating system designed from the ground up to use threads with a processor designed from the ground up to execute threads.