@OP. You can do a simple experiment but it will require the two different lengths of cable. Use one speaker, and a mono source and then listen to the two respective lengths of cable and ask if you can hear a difference. Ideally, you would do this blind with someone else swapping over the cables.
It's not a perfect test because, theoretically, any difference could be magnified in the final stereo installation - but for the setup you are proposing i.e. not your main system, it's probably representative enough.
BTW since electricity in a copper conductor propagates at approximately two thirds of the speed of light there is not going to be an audible propagation delay factor in what you hear coming from the speakers.
However! As you have access to sufficient cable, why not just put the excess cable on the "short" run under the floor.