I just found the directory on apple's forum.
\Users\YourUserName\Library\Preferences\SPORE Preferences\p_drive\User\Application Data\Spore\Games
FYI, I am running spore on my mac pro dual 3GHz Xeon with the fastest video card on the market for macs and this game still crawls and stutters, even with everything set to med. quality and 900x600! I'm guessing either my card isnt supported or its a bug. I hope it's something they need to fix in an update.
I wanted to do the opposite of what you want, since there probably wont be much of a difference in quality. And this way I can play a lot more