Really? I think +2 LA fits pretty well. 6 levels of fighter give the half-ogre worse reflex and will saves (and that's before factoring in the achaierai's racial ability bonuses). Remember, ECL 8 is where Charm Monster, Confusion, and Fear start getting used: the +5 the Achaierai is ahead of the half-ogre will save the party more than once.
Also, I don't see how you claim their ability scores are almost the same. The achaierai has 2 more strength, 2 more constitution, 4 more dexterity, 4 more wisdom, 2 more intelligence and 8 more points of charisma. The half-ogre will have a single ability increase from his class levels, but it won't make up for this.
You say it has CR 5 as if that is a reason to lower the LA. Ignoring that the CR system fails to do what it's supposed to do at all levels, almost all monsters have CR's lower than their ECL. A kobold adept 4 is CR 1 but ECL 4. A half-fey, widely regarded as a well-balanced template, has +2 LA but only +1 CR.
Also, AC. Counting the higher dexterity, an Achaierai will have an AC about 8 points above the half-ogre's, even if it has to pay double for basic armor.
Finally, the Achaierai is simply faster. Speed matters if you melee, and a 20 ft. increase in speed matters a lot.
So yes, the Achaierai is fine at +2 LA.