I definitely recommend a second drive for data no matter what. The wd black drives are nice.
I have 2 x 2tb wd green, 1 x wd black, 2 x ssd (256gb OS, 120gb project/scratch). the wd black is also where uTorrent downloads to, i've been keeping finished recording flac sets, lots of stuff. I'm actually in the middle of reorganizing data. Adding video into the mix really pushed my data needs over the edge. I have a lot of downloaded flacsets I can't bear to delete. This is all in addition to a separate Freenas box with 6 1tb drives in a raidz2 volume with about 3.7 tb usable that's nearly full. Just got a 4tb external usb disk. Crazy how big these are getting.
I can't recall if I tried the temp files on the OS ssd or not. The reason you may not want to is you risk contention with OS pagefiles and stuff like that. There's more going on there than just running OS binaries (which you probably know). My workstation is windows but OS X will be similar. Less performance impact of course since it's an SSD, but it's still there.
I also did it this way because the disks are relatively small. I didn't want to risk filling up my system drive if I wasn't paying attention and had a big project going on. The 120gb ssd was my first OS ssd and I outgrew it fast. 256gb is a good size if you don't store a lot of data there. And 120gb is more than enough to work on audio and video projects.
and what really cinched it.... I had the extra 120gb drive sitting there after upgrading
I had audacity reading and writing to the wd black as I worked recordings and using the same drive for temp files. there were several different configs that would have helped.