I P&G with my diesel and see probably a ~25% benefit vs steady speed. Granted, I also have a 3+ ton truck so it's not exactly the same as a passenger car.
You're right that diesels don't have throttling losses, most there is still a sharp dropoff in BSFC at light loads. That's due mainly to factors other than throttling, such as heat transfer and the theromdynamic effects of lower temperatures and pressures which reduce efficiency. So the concept of running higher loads as a means to higher efficiency is still valid with a diesel.
Secondly, one advantage that a diesel has when P&G is that since you don't have throttling losses, your "glides" are relatively more fuel efficient than a throttled engine when you're coasting in neutral (idling). That being said, idle FE can vary significantly from vehicle to vehicle.
Here's an old thread on the topic:
http://ecomodder.com/forum/showthrea...esel-9664.html