The iMac calibration is for (somewhat) ACCURATE color display---it may not necessarily be what looks the best out of the camera. To do any accurate photo work, you need an accurately calibrated monitor. If you don't have calibration hardware, the you can do a rough job in System preferences>Display>Color>Calibrate and follow the instructions. This is the iMac Calibration you want.
Apple RGB is a +color space+. It is not the same as calibration. Apple RGB is not commonly---if ever---used in photography. You should be working in Pro Photo color space in Lightroom. If not that, at least Adobe RGB. These are wide color spaces that are appropriate for +image processing/editing/manipulation+. A third, smaller colorspace, sRGB, is the web standard and you should convert to that for displaying on the web after processing.
All this becomes much more important if you print or send files to the printer. What looks good on you screen will be very unlikely to match your prints and that is where accurate monitor calibration is vital. Most typical labs use sRGB to print as it is what the average person uses---you need to check with them. If you want to print top quality photos yourself, you'll probably print from Pro Photo or Adobe RGB as they contain the widest range of colors.
Color management is a confusing filed at first (or maybe forever) and I am sure I just made it more confusing for you. You can find more info in Lightroom help files and other photography/lightroom etc sites.
Bottom line is that what looks good on your screen may not look good on any other unless both are properly calibrated. And if any prints do, it is from sheer luck. It depends to some degree on how critical you are though.
