I did not notice any significant performance improvements with HT. I run Linux (gentoo), and even that I enabled SMP and recompiled the whole system to notice the SMP, the performance increase is pathetic.
That being said, I don't run Apache 2.0 in threaded mode. The threaded mode is certainly a way to utilize hyperthreading capabilities of a HT enabled CPU.
Basically from what the articles say, the second virtual CPU would make use of any unused portion of the CPU to run a new thread. If the process is already loading up the core CPU then there is nothing left in terms of resources for the Virtual CPU to use.
It is just an efficient way of fully utilizing a CPU but does not necessary result in a speed bump. A HT CPU is effectively not 2 CPUs and should not be seen as 2 x 2.8Ghz, overall it is still a single 2.8Ghz CPU and you still be getting a maximum total of 2.8Ghz.