The difference between 20" and 16" is so small in all things that apply to you that I would not sweat any of the details. Sound will be about 3-5db louder but the concussion of the report will be significantly larger. You will FEEL it more, in other words, but it won't be really all that much louder... certainly not enough to make a difference in hearing loss. I'd take a pair of muffs with me to the field, but then I'm a metal worker and I'm around loud noises pretty much half the day.
Velocity: It will decrease, but the 30-06 and 270 are capable of cleanly killing a deer at 600 yards from a short barrel, no reason why your ranges would effect performance enough.
Accuracy: Believe it or not, it will be MORE accurate, but will certainly change your point of impact. More accurate because it's shorter and stiffer and your crown will probably be better too, no guarantees on that one though. There is a slight possibility that the load you use will cause a harmonic with any one barrel length that will make your load inaccurate, but you can get a different load if that becomes an issue.