I've killed at least 4 Bull Elk with arrows in the sub 400gr (down to 360gr) range. My hunting bows usually shoot about 270-280fps. I used fixed blade broadheads - specifically Muzzy 90gr 4 blade heads. No problems and will use again this year. Once I learned about yoke tuning, getting broadheads and field points to hit together has been a breeze.
Doing something doesn't make it the right thing. If it works for you that is great, however it is not advisable. That is a very light arrow specially at that speed.
NO Broadhead will shoot exact same POI as field points once you get past a certain distance. Close but never exact. Usually starts to open up at or around the 45 to 50 yard range.
