Hi Joe,
TG2 is using only half your CPU power because you've set it to use 4 cores. As you've found, right now TG2 doesn't scale well beyond 4 cores and gets slower once you go over that. This is probably largely because of overhead incurred by the OS, relative to Windows and Linux. As I said before, this is effecting a number of Mac developers. Hopefully Apple will resolve some of these issues, but at the moment they're suggesting workarounds which are quite a lot of work which is especially annoying when then Windows version, for example, requires no changes to perform well.
TG2 is currently at alpha level. When we move to beta we will be able to spend more time investigating performance issues and may be able to optimise things so the renderer scales better with more cores.
Part of the reason the 3D Preview is relatively slow is that it isn't multithreaded yet. Originally we did have it multithreaded but it lead to stability issues. I believe at some point we'll look into the changes required to make it stable when threaded, we know what's required but it will take some time. It may not happen prior to the final release.
TG2 only uses as much RAM as it needs. Sometimes that means it wants far more than you have available, sometimes it needs far less. There is no particular benefit to just grabbing all your RAM if it doesn't need it, in fact that may lead to your system as a whole slowing down. At the moment TG2 Mac will take up to about 1/2 of your RAM max, I think you said you have 8 GB and TG2 Mac can only use a maximum of 4 GB. This is the same for every application you use which is 32 bit and not 64 bit. After the final release we will be working on making both the Mac and Windows versions 64 bit and then TG2 Mac will be able to use all your RAM if that's is what is required for your scene.
Regards,
Jo