I agree with chewiefttc4. Good cells you will definitely notice the difference.
I recently GPS my VXL Rustler. Epic 3000mAh 6 cell sxs pack with stock gearing 25/83 ran 33mph. EP 4200mAh 6 cell sxs pack ran 37.8mph with stock gearing 25/83 .
Both packs were not matched. They were just the shotgun sport pack that was gutted and rebuilt to side by side config.