Since we had the 27-inch iMac on hand, we thought we'd see if OS X Lion introduced any performance changes. Despite more than 250 new features, we aren't aware of any fundamental changes in Apple's new OS X Lion operating system that would affect performance after upgrading. It still offers 64-bit processing, and with Apple's tight control over Mac hardware components, there's no reason to anticipate a learning curve like Microsoft's partners had to deal with when Vista upended the Windows driver model. Still, the version of iTunes that ships with Lion supports 64-bit processing for the first time. We also wonder about any performance impact from Lion's new auto-versioning feature. There's also the question of how well third-party programs will make the transition in these early days, since they might still be awaiting optimization. With the 3.1GHz 27-inch iMac on hand from our review this past spring, we took the opportunity to see if Lio...