Just upgraded to Mountain Lion and there is a serious memory leak in the new Safari causing it to use tons of memory. Just putting it out there in case it might sway some people toward the 16GB upgrade.
If anything, Mountain Lion should actually keep people from wasting money on a 16gb memory machine they don't need. All of the lag is gone for me - the machine is blazingly fast with 8gb. And that includes Safari, btw., which was painfully slow before.