Are you ending up with the right amount of preboil volume? If not, why not add some more sparge water? I fly sparge and always heat a lot of extra water just in case. Even if you batch sparge, I would think you could have the water ready and if your volume ends up short, you can do a second batch sparge with however much additional water you need.
Just as an FYI, I don't think you should expect to get great efficiency with a 16 pound grain bill for a 5 gallon batch. For my 9 pound grain bills, I routinely get 85% efficiency. I made an imperial stout with 19.125 pounds of grains and ended up with 70% efficiency. There was just not enough sparge water to really rinse the grains well.