To increase arrow weight, if a bow tunes with 30" .500 spine carbon arrows with 125 gr. heads would it be possible to go to full length .400's with a heavier head or is this unrealistic? If not unrealistic, how much heavier would the point likely have to be, ballpark?

I know there's a ton of variables... I have found that full length GT .400s shoot the same as GT .500 spine with the same size field points at 30". I hope this helps.

FYI... There's 1.5 lb of dynamic spine difference between full length 5575 and 30" 3555.

Tough to answer that question. This is what I have been doing the last couple of months. Started with a 500 spine at 30 inches, put 375 grains up front and shortened my arrows until I realized 500 was to weak. Went to 400 spine with the same grain front and started cutting a little bit off at a time. Finally the 400's are shooting like darts. I shoot a recurve that is 53 @27. I shoot the full metal jacket shaft, along with a tuffhead broadhead as I wanted a heavy front end for a moose hunt this fall,

.500s are 63# and .400s are 78#, a 15# difference, according to Easton's way of measuring spine. To answer your question.  Yes, you could probably add enough point weight to get .400s to fly right.  Have no idea how much though.

It can be done.You just have to decide how much overall arrow weight you want.There are lots of variables,but when I have jumped one,carbon spine group like that,the stiffer set weighed app.120-130 grs more than the weaker set,overall weight.All things being equal,shaft length etc,your point will likely be app 100 grs more.

Never have managed to shoot an arrow that had only a hypothetical spine...I shoot quite a few bows with quite a few different arrow and point combinations and when I want to shoot a heavier arrow there is always some aluminum involved...if one insists on sticking with carbon only construction things get trickier

DDave

FWIW I've found that carbons are WAY more sensitive to length than point weight.  (Conversely I think wood shafts are quite sensitive to point weight).
If switching to 400's I'd start full length with the same 125 gr. points.  If they show stiff, add point weight before you start cutting them down.
What are the specs of your bow and draw?  I use 400's on most bows ranging from 50-60 lbs with the variables being cut short or past center and usually use 300gr. Up front (100 gr inserts and 200 gr points).

I probably should have provided that info up front. My bow is a Shawnee w/recurve limbs, 60", 46# @ 28"(3/16ths passed center), 5/16ths carbons.  I'm drawing 29" so probably pulling 48-49 lbs, which is starting to get into that 'tweener' range.  I can live with the 500's and 175gr points.  I've killed a bunch of deer with them and sometimes short draw in the excitement, so a slightly weak shaft is in my favor, lol. I thought my draw length was always 28" until I discovered quite by accident a couple years ago when I switched to more of an "expansion" release coupled with shooting more when the weather gets warm, that it had increased to 29.  Admittedly, it may have had more to do with my form modification than the extra 3 lbs.  When I re-checked the tune the .500's had started to show slightly weak, necessitating the drop to 125 gr.  I've got another bow (same pounds, but higher performance) on order and was just toying with the idea of trying .400 spine set-up for it. I know with a new bow I could be looking a different shafts anyway, but it's the same bowyer (ILF riser)and centershot will likely be identical.

Been thinking of doing the exact same thing. I believe if I go to 300 grains up front of a 400 spine Carbon GT, Ill be fine.

I have a 62-inch Shawnee with 50# longbows limbs at my 28-inch draw.  It likes Axis .400s with 275 grains up front.  .500s are a bit weak for it. You are close to a twiner, but I think the .400s would  work better for you.  

BTW, I don't know what bow you consider higher performance. The Morrison Shawnee is about as high performance as they come, with carbon/foam limbs at least.