WHOA! There are a couple things going on here.
First, your issue. It is true that the gas neck on the Z tank is a little touchy. Often times the pump shuts off to early because of the neck shape. However, over time you will get used to how to properly place the hose in the neck. I don't have problems with this anymore.
If for some reason your are running until the empty light comes on and you can NEVER get in more than 15 gallons, then you have a secondary issue. One of your fuel sending units may be bad. Yes, the tank has two sending units and one pump because there is a hump in the middle fo the tank to let the driveshaft pass by. I had a sending unit go bad on me. Over time, the empty light would come on and I would get less and less fuel in at fillup. Eventually, I was only getting in 12 gallons. The test to determine if the sending unit is bad is obsurdly easy, so a dealer will find it if it does exist. Another side effect is of a bad sending unit is a moving fuel guage. Lets say your guage reads 1/4 tank. Then you turn a corner hard and it now reads empty or 1/3 tank. That is a moving gauge. This is caused by the fuel rushing from the side of the tank with the bad sending unit to the side with the good unit or vice versa. Just think about this and keep it in the back of your mind just in case.
True, DTE and the empty light are based on 17 gallons, but the tank is 20. Furthermore, there is a really, really, really good reason for this. The car is built for racing. The tank is even baffled to help keep gas near the fuel pump pickups. However, no amount of baffling will help you if there is not enough fuel to trap in. So, the computer "fools" the driver into getting more gas early. This keeps you from taking a hard turn with too little gas, uncovering the pickup and running lean. Of course we all know lean means PING, CLUNK, BOOM!
As a side note, this is also why most race cars and some street cars (i.e. MacLaren SLR) have a surge tank for the gas. You can run the main tank as empty as you want and ditch the baffles. The extra surge tank will prevent the gas pickup from ever coming uncovered. However, it does require two fuel pumps.