I don't think regardless the power I could ever be convinced of running anything larger than a good quality 16guage to any speaker.. Even a sub. I've heard 16 and 18 guage runs in good home systems which due to the quality of the cable and the better terminations, literally spank thick 12g lamp-chord. BUT, I do NOT condone spending silly money on it... Canare 4S11 at about a buck per foot is one of my favorites for 4-conductor biwire runs, for example.
Also, just running a single strand straight from amp to driver without any plug-terminals in any guage is going to be marginally to far better than having a bunch of harness-plug points between A and B... Even if you stay at 20 guage. That's probably your biggest benefit, I'd say.
Here ya go.. Kimber 16g 4-conductor.. good for two or four drivers..
Kimber Kable - KWIK-16 In-Wall 16-Guage Speaker Cable-Audio Advisor