1) your highs amp doesnt effect your cars power to drop much if any2) a cap wouldent solve your problems even if you added one
3) your system is still rather small to be having voltage issues
4) might want to consider a BIG3 type cable upgrade,its good for all cars
Rebuttal:
1.) I have roughly 400 RMS feeding into the mids/highs, which cover a much higher range of frequencies than my substage. My substage is pulling about 400-450 RMS. When my mids hit, my lits dim, when my sub hits, my lights don't dim. A higher frequency draws more power, even though it is easier to move the smaller speaker. (I may be wrong, but that seems to be what is happening in my system)
2.) Since my mids were causing dimming rather than long bassy farts, a cap would have done pretty good, as it was the short burst mids that were doing the light dimming. I also believe it is an alternator regulator issue, as the voltage itself didn't drop much during the mids, even though the lights did ( I have a constant voltage reader on the battery post and on the HU)
3.) 850 RMS possible to be used at my tuning stage on an allotment of 50 Amps (Car consumed about 20 Amps for regular systems). I believe that is borderline for my alternator before an unsafe voltage drop.
4.) I did one better, a Big 7. Yes, that's right, I used 7 4 gauge wires instead of just 3. My engine bay is a network of grounds and power wires, all covered in anti-corrosive grease to reduce risk of resistance //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif
At full tilt, I have a voltage drop of about 1.5 for loudest/hardest hitting songs, and major light dimming. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif