I am using 100 grain Montec G5 fixed broadheads. My set up is as follows:
Mathews Drenalin set at 55 lbs.
Beman MFX Realtree 400 w/ 2" blazers

Right now the arrow w/ broadheads weigh 405 grams.

My questions... if I change to a Montec G5 at 125 grain will it help my front of center % for better arrow flight?
How much penetration difference will it make?

I don't know what "point of center" is. It'll help your FRONT of center.

It'll also have implications on your dynamic spine. Whether they are negligible or not will depend on many things (the key's gonna be how close to "marginal" you are to optimal spine, now).

You may not see a discernable difference. It'll likely show up most in long range shots (beyond 30yds). Inside that, you may not even have to make a sight adjustment.

I shoot 220 grain broad heads. Total arrow weight of 543 grains, giving me a total FOC of 21%. I love the pentration. Pass throughs 100 of the time.

I will take measurments today and determine my FOC %. Then I will consider the heavier broadheads.

220 gr broad heads?? What are you shooting?

I shoot the same heads. He is shooting muzzy phantoms.

I wouldn't change heads unless your spine need to be weakened.

OK... The measurements are:
arrow length w/ broadhead measured from the nock to the the tip is 29 1/4
The center is 14 5/8
the balance point is 2 1/2 difference
so the FOC is 8.5%

SO... how much difference do you think a 125 grain broadhead will make?

Measured from nock to tip of what?

to the tip of the broadhead