a capacitor will only raise the peak watts and then only a marginal amount (maybe 5-10%) capacitors are only meant to smooth out the voltage dips and aid in reducing light dimming and alternator stress.

You will never regret extra power. The L7s are somethign like 350wRMS per channel, so a 400 watt amp will not push them to anywhere near what they are capable of. the 750 is pushing it.

You can always go ported, you just need a custom built box that will optimize space and take into acount the T/S specs for your subwoofer. You can build a ported box in an equal space as a sealed with a noticable gain in output without sacrificing a large amount quality. Especially if you are looking more for loudness than perfect song reproduction. (which is the only reason you would be using kicker speakers)