It won't hurt you to get a thicker gauge, but you're making more of the damping factor with regards to the wire than there is. The damping factor is dominated by the resistance of the voice coil. Until the resistance of the wire begins to approach that of the speaker, it's a non-issue. Figuring you've got 8 ohm nominal impedence, the resistance of 50 feet of 12 gauge is not all that much. If you're running in the wall, and you're still concerned, simply consider running two lines of 12 gauge and joining the minuses of the two separate runs together. Same for the positive ends. This'll give you an effective 9 gauge run of wire.
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