Well, if you use a thinner gauge wire, higher resistance, than what is recommended a few things could occur. Power could be lost in the wire, and the speaker would not play as loud. So you really don't want to go beyond the length specified.

Here is a great website that explains all of this, the wire table and the next few paragraphs explains in more detail.

http://www.roger-russell.com/wire/wire.htm

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