Unless you have a VERY long speaker run, or are doing a major upgrade in quality of the cable, it is unlikely that you will hear any difference between a 12 gauge and an 8 gauge speaker cable. If your speaker cable run is fairly short -- say, 15 feet per cable or less -- the difference in the resistance, capacitance, and impedance of the 8 gauge vs. the 12 gauge will be inaudible. (As an example, I use a 25-foot section of 12 gauge cable for the surround speakers in my HT system, and it works fine.)
The only other factor that occurs to me that might cause an audible difference is how the cable interacts with the power amp. If a problem occurs here, however, it is usually not due to the gauge, but rather a specific design configuration that causes the amp to oscillate.
To reiterate: unless you are going to a very long run of cable, or plan to substantially upgrade the quality of the cable, there isn't much reason to move to the larger size.
The only other factor that occurs to me that might cause an audible difference is how the cable interacts with the power amp. If a problem occurs here, however, it is usually not due to the gauge, but rather a specific design configuration that causes the amp to oscillate.
To reiterate: unless you are going to a very long run of cable, or plan to substantially upgrade the quality of the cable, there isn't much reason to move to the larger size.