03ExpyMG, for you, I would say doing this mod is going to be important. The stereo system alone can be pulling upwards of 200 amps. So, you start adding in other truck loads and you are going to have your alternator maxed out. So, upgrading the wiring all the way around is going to be good and should net you even more volume (due to the amplifiers not having to work as hard and being able to maintain its internal voltage higher).
It is nice that the shop upgraded the wire from the body to the battery, but keep in mind that the alternator is the source of power, so, the current has to return all the way to the alternator. So, upgrading battery to block and body to block will minimize any voltage loss there. Just to put this into perspective, even if the wiring added in 0.01 ohms of resistance (all wiring has some amount of resistance), with your stereo cranked up, you are dropping potentially 2 volts in just your wiring (assuming all other wiring was perfect). So, instead of your amp getting 14 VDC, it is only getting 12 VDC. This is where the difference between ok wiring and higher end wiring can start making a difference.
If you need to start knowing more about wiring, let me know. I will do what I can to explain things to you.