Efficiency actually won't be that big of a concern. It seems like 90% of your monoblock sub amps nowadays are class D topology. Class D yields somewhere between 70% and 90% efficiency. That 20% difference in efficiancy isn't going to make or break an electrical system. Besides, most manufacturers overrate their amps efficiency anyway. So, throw efficiency out the window. It's not an important spec. With that car, I wouln't even consider anything over 500 watts RMS if you want to do nothing to upgrade your electrical system. Going with something as large as a 1000 watt amp will require an upgraded battery and upgraded factory grounds at the least. Anything larger will require a high output alternator.
This all depends kinda depends on your taste in music. I assume you listen to rock or rap, in which case will be the most taxing type of music you can listen to, so this is where my recommendations arise from. If you listened to primarily classic rock, classical music, country, something a little more tame, you could get by with a larger amp, but I doubt that's the case.
A 500 watt RMS amp will draw upwards of 50-60 amps at full output and prob slightly more during peaks. A 1000 watt amp is likely to draw twice that of course. How long do you think your factory 85 amp alternator is going to allow you to get away with more than 120 amps of current draw from a single electrical accessory in your car?
Stick with the 500 watter or less.
