Straightarrow, when you switch from field points to broadheads, you've changed the total length of the arrow, which moves the arrow's balance point. You've added to the arrow's frontal surface area. You've added wings to the front of the arrow which contend with the arrow's fletching for control of the arrow. Too many changes to assume one can tune a bow to perfectly shoot both at the same time.
I agree with a lot of what you said, but a few things don't ring true. The balance point remains in the exact same spot if the broadhead weighs the same as the field tip. I've actually found the balance point and marked it with a felt pen and when switching to a broadhead, had it balance on the same mark - repeatedly.
If I have a bow where broadheads impact with field tips at 20 yards, 40 yards, and 60 yards, what could possibly be out of tune? If center shot was off, they couldn't possibly impact the same at all those varying distances. The same would be true of nock height. When a broadhead starts to plane, it will be off by even further amounts at longer ranges. If your broadhead didn't impact the same as the field tip, it would mean that your individual sight pins would have to be adjusted out of plumb.
I have no problem getting broadheads and fieldtips to impact in the same general area when I tune my bows. I've never considered this to be a sign of an out-of-tune condition. I don't have to mess with individual pins. They are all still in the same plumb line. Arrows fly to where I'm pointing upon release.
Your point about only being able to tune to your shooting ability is a good one. However, I don't quite understand tuning to group size. I've never seen anyone measure their groups. How would one know if at one adjustment point, group size was 3.125" or 3.375"? It would take painstaking measurements that would unlikely be very accurate.
Here's how I do it: For centershot, draw a plumb line on the target. Adjust center shot so the same pin hits the plumb line at 20 yards and 60 yards. If you can't do this with a broadhead, it means that your arrow is not tuned correctly. Spine is off, FOC is too low, broadhead is not on straight, arrow has inconsistant spine, or some other problem. I am always able to make whatever arrow tuning adjustment that is needed to get the broadhead flying true.
For nock height, I tune with a bareshaft and field points to get the ideal position. I use string nocks, so I can make very fine adjustments on nock height. One turn of the string nock equals about 1/32" adjustment. Sometimes, I only use 1/2 turn of the string nock.
I remain unconvinced that impact will be significantly different on a setup that is correct. I'm referring not only to the bow's tune, but arrow spine, FOC, spine matched arrows and tuned broadheads. Maybe it has something to do with how I tune everything. I use stiffer than normal arrows, lots of helical feathers, very high FOC, heavier than average arrows