Remember that a non-Warforged fighter will be able to take advantage of their armor proficiencies. They will start, probably with a +4 or +5, and sooner rather than later will buy/loot full plate. The will then have 2 (or 3) feats to spend on Weapon Focus, Power Attack, Iron Will, PBS etc.
A Warforged fighter will surely take Adamantine Body (assuming they aren't an archer type). They will have a great initial AC at the cost of a feat. They will have one feat left over to improve their capabilities. Within a few levels, the Warforged and non-Warforged fighters both have a +8 AC but the non-Warforged are up by a feat (arguably two if you consider that the non-Warforged can use their Heavy Armor feat). The non-Warforged are also able to take off their armor and use, "out-of-the-box," armor they find. The Warforged always have to take armor penalties and have to hire someone to improve them.
Certainly there are advantages to having your armor built in, and the Warforged have many other advantages as well, but in my experience they are balanced just fine, and if you include non-fighter classes have a distinct disadvantage.