increasing ionisation of sulphuric acid as an electrolyte
chemicals improving conductivity
These are two different things.
Ionization is a matter of acid dissociation constant and can't be changed.
In diluted solutions to increase conductivity one can add more ions, but in concentrated solutions (like the acid in acid battery), this can be counterproductive, because of many possible side effects. Plus, any new ions added may interfere with the lead acid battery chemistry.