Yes, this is something that will for sure make them better.
Not while ago in the beta group, I wanted to change the mercenary boost upgrades to have less effect on unit stats, and focus more into their abilities.
This can also reduce their cost from 300 (which is expensive) between 150-200 maximum, and will allow the players to invest more minerals into something else instead.
So the mercenary boost upgrades, will become something like "optional upgrades" that can boost the mercenary's ability.
For example Marauders will get bigger AoE for their consussive shells, stalkers a higher blink range, roaches a faster burrow and unburrow time, etc. etc.
Better balance can be achieved by removing from these upgrades any parts that affect the unit's performance greatly, such as giving faster attack speed or higher weapon range.
Because a player can say goliath sucks and are not good, but looking at the compound the player didn't bought a single boost upgrade, something that affect's their balance highly.