I crunched a few numbers and realized that the removal of armor from the eidolon doesn't fix the AC issue, and the best AC comes from a small eidolon.
Huge: +5 NA, but -2 Dex mod and -2 size for gain of +1.
Small: +1 Dex mod, +1 size for a gain of +2
Start with a serpentine form for 16 Dex. Small makes that 18. HD increase makes that 22. Automatic level increases make it 30. Evolutions put it up to 38, for a +14 modifier.
Natural armor starts at 2, and goes up to 18 by leveling. Evolutions kick that to 26, since that evolution is only once per five levels. Eidolon has 9 feats, all of which can be Improved Natural Armor, pushing just the NA to 35.
Dex mod, nat armor, and size bonus total to an AC of 60, with a respectable touch AC of 25.
Wearing armor isn't the problem, and should be allowed with a clause that says it overrides the Natural Armor.
The problem is the natural armor. The inherent leveling bonus is far too high, and the evolution is too cheap.
Is this an extreme example? Undoubtedly. But since we seem to be nerfing things based on those types of examples, I felt it was appropriate nonetheless.
I do not deny that the AC can be pushed far too high, but I think changes, however temporary were knee-jerk reactions and missed the real problem.
No, the normal armor was a much bigger problem because it required virtually no investment from the Eidolon. Compare 2 evolution points for 14 AC to what your Eidolon invests. The problem wasn't getting a crazy-high AC, it was getting a crazy-high AC with minimal investment, meaning the Eidolon could have crazy-high AC and still be an offensive threat.
Aside from the feats, which I only added to further prove a point, the investment wasn't that big. Raising Dex isn't just an AC booster by a longshot, especially if one of those feats becomes Weapon Finesse, and I still have plenty of points for some natural attacks. Ditch the feats and make it Medium and it's still got an AC of 49. 5 points for 10 AC is not enough. Armor may be cheaper, but it actually requires you to invest gold in it, taking away from gold you could be spending on your gear. And the armor can be broken. The game is balanced on that gold expenditure also. So 2 evolution points for something you have to invest gold in and that can be broken just doesn't seem to cheap to me. The 16 NA and 4 points of Dex to AC you get for free is too much.
The problem with what you are doing, is you're pumping everything into AC, it will have no offensive. Frankly, a giant's just gonna pick your lil Eidolon up and toss it over a cliff. Anything would just walk right by it and ignore it. Having an extremely high AC is nothing unless it has some kind of bite to it. If you want to make an example, show something that can not only avoid being hit but can take down a lot of stuff.
I am well aware of that. I am simply presenting the same evidence that others have regarding balance issues. That said, this eidolon still has at least ten points to spend and 16 Str. Is it a melee powerhouse? Definitely not. Is it completely useless with 10 tentacles, a bite, and a tail slap, especially if I take Weapon Finesse, meaning most attacks will hit? Far from it. And if we don't give the summoner any evolution points, we can give it improved damage for all three attack forms (and Medium ups damage die and brings Str to 20 for the cost of 2 AC).