The output of the alternator will be lower at lower rpm's. You will have a little dimming and dipping of the voltmeter in the dash if you are hammering the volume. Be careful not to let it dip too low, 10 volts is not good for anything. In other words, turn it down at stoplights and other stops where you are at idle.
With that said, I rerouted my engine ground to the case of the alternator. I used a ring terminal on the 0/1 gauge and put that into the mounting bolt on the back of the alternator. Insulating this lead is crucial to keep the supply and ground separated to prevent shorting out. See if that helps.
If you are still wanting more stability, time to buy multiple batteries to stabilize the voltage, but your alternator should have enough juice for your situation.
Also, a note on capacitors. I consider them to be a "cleaner/filter" of the voltage supply, nothing more. Thinking capacitors will supply more power is wrong. They are not a power source. They are a throughput device.
Hope this helps...