Yes, you will have a loss, but not enough for the human ear to hear a difference (unless you are a complete audiophile). I've seen many competition installs that maintained the factory speaker wiring without any loss. I've also installed hundreds of amps, a few that included a full rewire and I never heard a difference. In my old Dodge ram I had a Kicker 5 channel that was pushing around 75w rms to each speaker. After my factory wiring had issues (typical Dodge Ram broken wires in the door jams) I rewired it with 14awg speaker wire. I did not hear any difference in sound quality... and I normally hear the imperfections that most don't. The OP can do as he pleases, but the added time and cost just isn't normally worth the tiny difference you may have.