There's a few extra complications involved in using a thicker wire... One factor is the weight, which is already considerable with the bulky power supplies. Another is having to connect a 10-gauge 4-conductor or 8-gauge 2-conductor wire to a small C13 connector, which is the only way to plug the power cord into the LEDs (I know, that was a mistake).Why can't you just change the wire size? The cost is not that significant.
I've seen 25 ft of 2 conductor #12 wire for 10.00 on ebay.
With #12 wire you will only lose about .75 volts, I doubt if you will even notice the difference using a 12 volt supply
Another point, #18 wire is nominally rated at 10 amps max. and at 12.3 amps the wire will dissipate 38.5 watts or almost 2 watts per ft. Use #12 wire.
SG
Please though, can you link me to wherever you found the maximum current ratings for different STRANDED wire gauges? I cant find a definitive answer because some ratings are for a specific voltage drop, while others are for "power transmission" and the like.