Hi,
I typically run TG2 with 16 threads without problems, although I'm not often rendering particularly intensive scenes. It will often render somewhat faster with less threads, 8 or 12 for example. This is because even there is overhead from running so many threads which can end up slowing things down.
With the 32 bit version I don't think running with 24 threads would be such a good idea because even with a smaller subdiv cache, for example 100 MB per thread, that still ends up being 2400 MB which doesn't leave a lot of memory free for other things.
I think you need to experiment with scaling back the number of threads and then with the subdiv cache amounts so you can find the best performance balance for the typical scene you render. There is no one answer to this for everyone.
When the 64 bit version is available you'll be free to allocate more memory for the subdiv cache, but until we can really optimise the threading overhead you still might find that 24 threads is too many.
You might be interested to know that the threading overhead is greater ( i.e. ends up with slower rendering ) on OS X than Windows.
Regards,
Jo