These recommendations might help - http://www.lowertheboom.org/trice/safedblevels.htm
. The required hearing protection is obviously going to depend on your environment.
I think that overkill is when you're wearing so much protection that you can't hear range commands or warning sirens. For example, there are a few members of my local range who have some existing hearing loss and now double plug. They usually make the effort to shoot near the buzzer and pay attention to the line, but sometimes they miss the cease fire and will continue shooting because they can't hear the signals. So that's a consideration.
The Leight Sports are decent muffs, I have two pair. But as FlyFish stated, they don't have a high NRR. And they're shallow, so plugs that stick out too far, or have a stem will poke the inner wall. This is uncomfortable and tends to dislodge the plug. I'd go with a deeper muff for double plugging.
The effectiveness of both muffs and plugs relies on the quality of the seal they form. If you're double plugging, and want the maximum protection, don't use plugs w/ a joining cord as this will interfere with the muff's seal.