New mod version uploaded and is available from the first post.
Currently champs aren't strong enough for that to be required. Would make them obsolete in the current balance, but maybe in some future rebalancing.
I suppose it wouldn't be bad to do so and it is already done for some units (swordsmen in vanilla, swords and all champion infantry types in my mod). Additionally why change something that works perfectly fine considering that the changes will require much more testing and one of my goals is to keep the changes as uncomplicated as possible.
I have increased cost of champs slightly in the newest version. 4 armor levels is 34% armor, I'm not exactly sure how that would work out to 52% extra hp but maybe I'm missing something. Additionally armor levels don't scale linearly. 4 armor is 34%, level 5 is 41%, level 6 is 47%, 7 is 52% and so on. Normal spear cav have 4 base pierce armor and champ spear cav have 8 pierce armor, that is 34% vs 57% pierce armor. With armor upgrades those percentage differences become smaller. For the champions having two times more health, it is half true. Champions have 2x more health of phase 1 units, but phase 2 gives a 20% hp bonus to just citizen soldiers and phase 3 gives a 10% hp bonus on top of the phase 2 hp value. So the difference becomes quite a bit smaller. There are a few more things disadvantages, those are that champions give more loot and experience, and that they can't work or build. So champs don't work out to be 6x better in the end. 0ADs citizen soldier utility makes things on the balancing department a lot more difficult.