Pointed at D2846.
"special_war_horses" gives the player a +20% health for all cavalry (including champions). Cost is not equivalent to its gain.
"mace_silvershields" must have research time equivalent to similar tech.
My proposal:
Increase the research time to 60 for upgrade_mace_silvershields;
Pointed by @Nescio, Nisean horses were used for heavy cavalry, so it only makes sense to affect champion cavalry. Cost 350f 350m (value equivalent to other techs).