ACOGs in general are not precision optics. They have VERY good glass, and are extremely durable, but are designed as a combat optic, for shooting humans under adverse combat conditions.
If you are looking for a precision optic to shoot small groups at several hundred yards, you are making a poor choice.
If you want a bulletproof optic for fast shooting, then an ACOG is an excellent choice.
$800 isn't a bad price for a used TA11.