Forbidden and geepherder are both well known here and I believe are 100% correct. If the Alternator can supply the needed current, the voltage should remain stable and healthy. Batteries provide the under supplied current for short bursts but still must be charged. If you are generating enough juice to power everything there should be no dimming and be able to run A/C, lights, whatever. Proof is in the voltage, plain and simple.
If you can't supply enough power for demand, You can ground every square inch of metal twice and the problem will remain. Test bench, fiberglass boat or store display you must generate enough energy first and foremost. Next you must have the capacity to get the energy where it needs to go. Using the correct gauge wire should go without saying. The Big three makes up for manufactures saving weight and money. No reason not to do the Big 3.
This is how the use of cap(s) was explained to me: Cap(s) aka Stiffening Cap(s)are small container(s)used to top off a bucket(battery). They keep the bucket topped off and fill in for occasional leaky drips (enter the stiffening part of the term Stiffening Capacitor). The Cap dumps what ever it has for as long as has to keep things full, but it empties within fractions of seconds. Ex: attach an incandescent bulb and see how long it lights, recharge cap again, how long does it light? No longer than the first time. All the Cap can do is get more water from the bucket and try to add small amounts back in. Despite what marketing companies claim, a cap can't make energy or be substituted for an inadequate source. The battery supplies what the alternator can't (the remaining water in the bucket. More battery capacity = bigger bucket). Solution, get a hose (HO alternator) that can keep the bucket(s) continuously filled. If the supply keeps the bucket filled under load, a Cap could be useful to stiffen the voltage. If you are are serious SPL competitor it might be worth the time and money.
You are fortunate you own a GMC vehicle which means there are plenty of readily available HO alternators. Installation should be straight forward and simple.