For the most part, the average user won't notice any performance degradation from using Aero, or the Windows 7 themes. However, if you have a lower end graphics card, or tend to use alot of graphics intensive applications, you can benefit from turning off Aero and the Windows 7 themes.
In other words, unless you've got a graphics card with less than a 3.0 rating in WEI or run alot of gfx apps, you're fine.