Safari 6, on Mountain Lion, is actually MUCH more memory efficient than old Safari on Lion.
On Lion, the main memory killer - Safari Web Content - would not only take up hundreds of MB of memory, but also would not release on their self over time, even if you close down all the Safari windows, unless you cmd+Q safari, or let the thing go asleep for quite a while. The inactive memories would also not release by themselves unless you force quit the things like Safari, VMWare, or you use purge command in the terminal.
On Mountain Lion, I've noticed that the issues above are mostly gone. The most easily noticeable thing is Safari Web Content will not stuck there forever. For example, after closing down all the windows of Safari, the Web Content - which would take let's say, 300 mb of memories - would gradually decrease by themselves in a few minutes until the progress is gone. Also, I've also noticed that the blue Inactive Memory would suddenly diminish, which proves that the OS is actively releasing Inactive Memory.
All in all I find that memory management is much better in Mountain Lion, despite the fact that it introduced tons of features that are supposed to take up more memories (notifications, better safari scrolling, safari tabs browsing, etc).