I looked at some so called scientific studies. It said that many of the whistles worked way beyond a deers hearing range. Plus the decibels are only just louder than a car. So even if it was at a frequency the deer could hear, if you are louder than a car. aka after market exhaust the deer wont hear it over the racket !
Your exhaust may just scare them more than a whistle. My risk reduction technique for deer strikes is to clench my butt cheeks tight, pray and look around furtively as I pass through Savernake Forrest. I did whack a MonkJack Deer in a Type R once and they are very heavy little fekkers. Just like Rudolph I did get it airborne.
