Yes, It allows you to do all effects that you could do that require multilayers
As for effects, depends on the version.
If you are having to render each time you add a clip to FC, then I suggest you first convert it to something like DV format with Linear PCM audio at 48kHz. FC likes that. This will not help with having to render effects.
If your version of FC has RT Extreme, then depending on your hardware and type of effect, you may not have to render until you export.
You haven't given enough information for further discussion.
How do I convert to a .mpg to DV format