I have always been a fan of string through body. I have read numerous articles on how to achieve sustain. The bottom line is the ability of the string to impart its vibration to the body of the bass, setting up a feedback loop between the two.
The way the bridge and strings fasten to the body is everything in this equation. Strings through the body is one way to achieve good vibration transfer. G&L uses a saddle lock bridge with a slot that buries into the bass body. If the bridge is not string through body, it needs to be heavy mass and well secured to achieve good sustain.
The string inclination is also very important. For this reason, some feel the increased angle required for string through the body eliminates the benefit.
The P7 bridge design changed a couple of times in the Gen 2. Mine has the conventional barrel saddle. My Gen 1 P7 had the high mass bridge. I honestly can't tell a difference. They both had sustain for days.
When you play, if you can feel the body of your bass vibrating with the string, you have a good one.