No, off. As I understand it this is only meant for using Diva without anything else which seriously consumes CPU at the same time.
Cubase (read: Windows - if I’m not mistaken Cubase utilizes the Windows thread scheduler) is very efficient at spreading the threads, which are caused by the tracks, over the available cores (physical and logical).
So I keep this setting in Diva off, because otherwise it would interfere with the scheduling and create all other kinds of unnecessary problems (a DAW MAY assume thread states which could, theoretically, be thrashed by interfering threads - but this depends on how clean the design of Cubase is, which is something I don’t know).
I think it works like this:
For each track, Cubase starts a new thread. Windows then issues this thread to the CPU core with the lowest load, filling up the physical cores first, then the logical cores.
Send-FX will create synchronization issues, of course… because multiple source threads must complete their work before the FX tracks can start theirs. But in my (balanced) example this should be ok.
I see a performance gain (in theory) with HT, because of inefficencies in utilizing CPU resources when the CPU resources have to wait for I/O (including cache / memory) or architectural state (which includes the pipelines, which are efficient, but not perfect, even in a 4th generation i7).
From a theoretical point of view, HT is the way to go. Of course, “the devil is in the details” (as we say in German).