Okay a 100 grain insert is longer and I realize it will act to some extent as an internal footing.  All things being equal ( insert and point weight) say both the shorter stock aluminum and the longer brass are in the 275 gr finishedd range how much difference if any can I expect in arrow flight?

Others might chime in but in my experience, not much difference.

For example, a 200gr head and 25gr insert....seems to work the same as 125gr head and 100gr insert. They shoot, tune, etc the same for me.

There was a big difference between a 100gr brass insert and 100gr with the  old GT weight system (much much longer internally and had a bigger effect on spine)

Not enough difference to notice. 

TEchnically a longer insert will make the shaft stiffer but not to a degree that I have ever been able to notice

I generally use a 25 grain insert and a 200 grain point, but since I can't find a judo heavier than 125 grains, I use a 100 grain brass insert and don't notice any difference.

In my experience and opinion, if the average Joe can tell the difference then your arrow is not very forgiving and on the edge of tune either borderline stiff or borderline weak.

With ol goat, most of us would not be able to tell difference, if any, at the distances we shoot.