You got the right idea, on the wiring, but backwards.
8 ohm is greater than 4 ohm. Parallel does lessen the load on the amp. Parallel'ing two 4ohms gives you 2ohm. Series is the opposite changes 4 to 8ohm.
The cap actually increases load on the alternator. The cap provides a voltage discharge faster than a battery is capable of, for those transient spikes in the bass (hard kick drum). The bad part, though, is that it charges as fast as it discharges, so it sucks up all that juice from the charging system (alternator and battery) leaving the rest of your car to survive on what's left. This IS better than no cap, because in a no cap instance, the amp is trying to pull that sudden voltage from the battery (that's where it's connected) which can't supply it as fast as needed, so it pulls the 12v from the battery and the other 2.4+- @ 40-60A from the alternator. Then, when the amp is done, the alternator has to go back and start charging the battery again, running the car, and still provide the amp with rms power.
Kinda wicked, eh?
So when the amp is done the alternator has to go back and start charging the cap again, whats the difference?
Now, to argue your contradictory remark; if two 10's did have more surface area than one 15, then they would play lower as they're moving more air (hypothetically). But seeing as they don't play lower, that's a dead give-away they don't move as much air. BUT, put them in the correct box, and they can play as low, until you put the 15 in the same style box. Make sense?
I have to disagree with this statement, let me explain. If you have two paint rollers, a 10 inch and a 15 inch, and you have blue paint, which roller makes the paint more blue?
Neither, but the 15 puts more paint the the wall. Taking a quality lets say a RE Audio SX 10 and RE Audio SX 15, they will play the same freq range but the 15 will do it louder.
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MY AUDIO INSTALL
Sub Amp Kenwood 7252
Mids and Highs Amp Kenwood 7202
Front Speakers Infinity 6020CS
Rear Speakers Infinity 6020CS
Subs 2 Infinity 1052's
Deck Kenwood KDC-X792
I have to disagree with this statement, let me explain. If you have two paint rollers, a 10 inch and a 15 inch, and you have blue paint, which roller makes the paint more blue?
Neither, but the 15 puts more paint the the wall. Taking a quality lets say a RE Audio SX 10 and RE Audio SX 15, they will play the same freq range but the 15 will do it louder.
And a 15 can play 15khz and a tweeter can play 5hz, good job.
You know exactly what was meant by the statement.
How about I add a word in there, "perceptively". In the correct box, two 10's can play PERCEPTIVELY lower than a single 15, until you put the 15 in the same style box designed for it.
Best Buy Autotech Installer 
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