Suppose you have two Beman/Axis 500's of identical length and spine (just because the weight of the HIT inserts is negligible). The first arrow tunes perfectly with the HIT insert and a 175 grain point. If the second arrow has a 50 grain brass insert and a 125 grain point, will it fly equally well or does distribution of the weight make a difference?

Try it and let us know.  

The brass insert arrows are a touch stiffer on my Bemans.

The added length of the brass insert does stiffen the shaft slightly only way to know for sure is to try them.

The heavier, all steel 175 grain arrow lands first because of the Earth's magnetic forces.

They should both work just fine and I doubt most archers would notice the difference if any in point of impact.  The difference in weight distribution is minimal.  Try it out and let us know how it works out.

Tim,
I appreciate the vote of confidence, but I'll defer to those whose form is consistent enough to discern a difference to conduct such a trial. I was just curious, as among locals I've asked the question, the concensus is the arow with total point weight in the tip would show slightly weaker than one where some of the total weight was from the heavy insert embedded in the shaft.

Yup, it does stiffen the arrow ever so slightly(brass insert) but as said above 99.9% of people could not tell the difference in flight. Shawn