Hm, ok I'll bite (also a Greek player). Please help me understand how the Companion is OP. Looking at each civ's unit bonuses (enl.), we have:
Greek: Stradioti vs standard heavy cav: 7500 vs 7128 HP, 270 vs 224 DMG
France: Hussar vs std. cav: 7800 vs 7128 HP, 258 vs 224 DMG
Ger. & Jap.: Special vs std. fusilier: 264 vs 200 DMG (Ger. gets 290 DMG after a victory)
Roma: Special vs std. fusilier: 488 vs 390 HP
Brit: Yeoman vs std. musketeer: 36 vs 31 DMG, Chinese: 38 vs 31 DMG.
So the Greek unit brings an added 372 HP and 46 DMG
French brings an added 672 HP and 38 DMG
Ger. & Jap. brings added 64 DMG (Ger. brings added 90 DMG after a victory)
Roma brings an added 98 HP
Brit brings an added 5 DMG, plus 1 range, Chinese bring added 7 DMG.
As a Greek player I typically bring 12 companions, so I'll use that number here as some use more and others less. The infantry-focused civs will often bring in 90-100 of their special unit. So multiplying the above numbers by 12 and 100, we get the following benefits:
Greek: Added 4464 HP, and 552 DMG.
French: Added 8064 HP, and 456 DMG
Ger. & Jap: Added 6400 DMG (Ger. added 9000 after a victory)
Roma: Added 9800 HP (10780 if you add 10 more units)
Brit: Added 500 DMG, Chinese added 700 DMG.
(These numbers are taken directly from
http://dominations.wikia.com/wiki/Troops, and don't include wonder/library bonuses that would be available to all civs.)
The British and Roman units are the only units that brings an additional ability (+1 range and +10% capacity), and German receive 10% for all units, not just their Infantry. Numerically, it would appear Greek and British/Chinese units are at the lowest rungs on the ladder in terms of the advantage they bring to the table here. What am I missing? At first I figured you might be thinking of your experience with Companions in Classical and Medieval ages, but their comparative advantages are even less during those ages, and any civ can bring heavy cav and achieve similar results during those ages. My experience during late gunpowder and early enlightenment has been that heavy cav (Greek or otherwise) are not the units you want at the core of your army. If you meant to say that heavy cav in general are OP in earlier ages, I might understand that (not really, see below), but to say that Companions are OP in general just doesn't make sense looking at the numbers.
Now to get up on my soapbox: this is a wargame with an extremely expensive nation building component. In the hands of an elite player, all nations should be *capable* of 5-starring every equal or lesser base, every time, with no more than 20% losses, without consumable items like mercenaries, blessings or tactics, which are available to help when attacking higher level bases. If we can't have that, the cost of upgrades should be reduced significantly. Nerfing any unit has only one important effect: it takes longer to accumulate resources, creating more pressure to buy crowns. Looking at it from this perspective, the notion of OP really only has any meaning where a unit allows an *unskilled* player to 5-star the base of an equivalent-level elite player with no effort. Greek cav have _never_ been able to do that.