8 AWG is a bit overkill for a phono stage, yes. That doesn't mean that it won't sound good, or that you won't like the effect.
Phono stages have an absolutely constant power draw (unlike power amps) so you don't need "uncompressed power delivery" either. For example, Audioquest splits their top cords into high-current, large gauge versions and smaller-gauge source versions for this reason. So instead of buying a massive gauge that's not needed for the application, you can move up the line into a smaller gauge that uses higher quality metals.
