Hi @n80 I have a family room system set up exactly as yours, with one speaker on each side of the fireplace and the components on the middle of the side wall. The long run is 10 feet longer than the short run.
I dwelled on this same question, and after speaking with several cable manufacturers got their same reply, that the different length would not be audible.
I chose to make my ables from Furutech FS Alpha 36 wire and Furutech spades from VH Audio. I left my homemade 10 g copper wires to the low end and ran the Alpha 36 to the high inputs. Sounds great and no extra cable under the floor, and saved cost over extra wire as well.