Most of the time, sealing the car as tight as humanly possible will increase the volume. As the sound will not be allowed to interact with the outside air. With sound you create compressions and retractions, you essensially pressurize the air. If opening the the windows creates more sound, why not put the speaker on the roof?
Resistance makes a huge difference. When the voice coil in the sub is cold is where the resistance will be less. Although music is dynamic, and the actual power will change with frequency due to the fact that the voice coil in itself is an inductor and the speaker is a motor and both of these have thier own resonant frequencies. When the voice coil is cold the nominal resistance for dc is approximetely 4 ohms for most speakers. Like all metal, copper has a heat coeficient, the resistance of the copper will increase with temperature at a rate of ~0.00393 per deg C.
So if the voicecoil heats to 200 deg C during use, the resistance will increase to 4.7074 ohms. So at 20 deg C the resistance is 4 ohms and the voltage output is 40Vrms, the wattage used by the coil would be 400W. But at 200 deg C the wattage used by the coil would be 340W.
Adding or subtracting speakers will change the resistance as well, less resistance will = more power. But can the amp handle it? 40V output from the amp on a 1 ohm load = 1600watts but at a 4 ohm load = 400 watts. How is this the same? I hope you have spare fuses.
To get more power from a higher load, you will have to increase the voltage. So you will have to turn the volume up on the deck, and at some point you will overdrive the amp into distortion which is bad for both the amp and the speaker.
Also, there is efficiency. Volume will increase at a rate of 3dB ever time the power is doubled. Take a 1000W speaker with a 87dB efficiency (1W @ 1meter) and a 500W speaker with a 92dB efficiency (also 1W @ 1meter). So using a log2(500)*3+92 = ~119dB for the second speaker. log2(1000)*3+87 = 117dB. To get 3 more dB out of any situation requires you to double the power. I forget the exact number, but I believe you can gain an extra 6dB from just adding one speaker of equal power.
To get the most amount of flux, you must have a large magenet as close to the coil as possible. If thin wire is used, you could save space by having little expansion due to heat, and increase the closeness of the magnet to the coil, although power handeling would be limited. Do the opposite with thicker wire to handel more power and you would have to wrap the wire many more times, increasing the gap around the voicecoil, decreasing the efficiency of the motor. Now you must use a larger magnet. Larger parts = an increase in mass. Then there is saturation, which I don't know as much about.
Basically what I'm trying to say is, buy more speakers, not more power. One extra speaker will make the world of difference. A lower power amp with more lower power speakers and some time to shop around can yeild the same results without hurting your wallet.
Next off, your lights dim in your car simply because of the alternator. The sensors adjust field windings inside of the alternator, and cannot adjust at the rate the audio does. So as the bass hits the voltage can go from 14.4 to 12 before they catch up. The field windings increase alternator life, if the alternator ran without these you could run the alternator at its fullest potential and it would burn out real quick or fry some belts from the excess torque it would need. If your really worried about it, get a capacitor.
Excuse my grammer.