I dont know how much the higher stats are an advantage tbh, since at that stat level a lot of mercs tend to overkill, especially range units. this leads to the weird thing about the cannoneers being that they are actually lesser abus guns but since they overkill less and cost one pop they are more effective.
after a bit of testing, I think the boost that they get is the same as the mercenary contractor actually, just staggered across the age ups.
age 4 mercs get only one boost when going to age 5 of about 30%
age 3 mercs get 2 boost totaling 50% when going to 5 ( 20% in age 4 and 30% in 5)
the only exception to this is currently the cannoneer since they are available starting age 2 so they get a total boost of 70%
This actually makes their mercs still lesser then the two euro merc civs actually since the age 4 boost is less then or equal to the upgrade cards that those civs can use ( german gives 20% in age 3 and swede gives 25% in age 4), Hausa has an upgrade card, but it only gives 10%. not to mention that Euro civs can have additional upgrades on top of that with arsenals and more universal combat cards. The only exception to this is the cannoneer since I don’t know how it interacts with the mercenary contractor
They get a free mercenary contractor in age 5 but like I said im not sure how much of an advantage that is unless you are in a merc vs merc fight, other civs can just grind down your army and you lose a not easy to get resource for nothing.
As for asian civs, they have more obscure bonuses that kinda already work with mercs so im not sure how you want to buff mercs for them tbh, also only china really uses mercs since the other two civs already have high stats unit in the lategame.
China has a total of 15% HP and 8% attack boost for all units from Western reforms and the brit consulate and also guaranteed acess to the second most powerful merc in the game imo the iron troops. also more obscure stuff like universal artillery cards ( not too relevant but very fun with bombards)
Japan has 7% attack boost from consulate, golden pavilion +auras ( both of which scales from current stat so they already scale pretty well) and also arsenal techs - In addition, Japan is the only civ that has mercs with a total of 110% shadow tech stat boost from the japanese consulate
India is probbaly the weakest here with +12% HP and 5% attack from the Shivaji tactic and the brit consulate
Japan is probably already fine, india for mercs tbh I kinda want to create a new elephant mercs and make it unlockable by India to take advatage of their unique elepant cards ( reduced cost, pop and 20% combat cards).
For china, I just want to be able to make manchus trainable together with iron troops. Maybe if you want to get further funky make one of the monk techs give addition 10% boost to mercs as well ( they are supposed to be repentant after all)