I don't use any sound protection in the helmet; it's just not necessary. It provides more sound attenuation than a regular headset; almost too much in my opinion.
The HGU-55/P might have some delay due to it being a current-isssue military item, but I wouldn't think so. Especially if you're buying a clearance item that's ready to go.
Gentex doesn't remove nape protection; they relieve the helmet there to ensure it doesn't get in your way when you're moving your head, looking for traffic maneuvering, etc. The nape strap is a critical piece of equipment that prevents loss of the helmet during violence, and also service as protection.
Note that the 55P wasn't designed as a crash helmet; it's primarily intended for high speed aircraft in which an ejection in flight is a possibility.