125 gr will do both. It'll definitely improve the FOC, but KE won't change that much. However, if your arrows are as underspined asI suspect the heavier head is only going to make matters worse.

The reason KE doesn't change much is that KE is based on both mass and velocity. A heavier head (mass) will slow the arrow down (velocity) so they kind of cancel each other out. It's not something I would even be concerned about. With your bow at 70# and 31" draw you're going to have enough KE to hunt anything walking on this planet.

And it doesn't matter how much KE your equipment produces if it hits the animal sideways due to bad arrow flight.