Not aware of any way to make the power automatically consistent through the run.

How hot is your motor (and perhaps ESC) getting? Hot motors tend to fade some, may be adding to the change you are experiencing.

If it's only a matter of more trigger pull, learning to do that may be more desirable than a larger battery pack. A huge pack can make a 1/8 handle sluggish. Many guys are running smaller 4S packs or even a pair of shorties without issue.

Could be a weak pack too, may be contributing. Lipos are generally pretty good for consistent power, don't hear this type of concern much.