If you can still afford a better sub there is no real reason to go to a midlevel sub just because you can't max it out power wise. All that really means is you'll have a very reliable sub that will stay cool easier, sound better and likely still get louder off of 700 watts than the 700 watt rms sub would. Just because a sub can handle more power than anther sub doesn't make it less effecient. With the way power and output scales, if it was, there would be NO reason to ever get a higher powerhandling subwoofer. Think about it 3db is doubling of power and 1db is a pretty hard difference to hear, very hard. If you lose 1db of effeciency to gain 2x the powerhandling, you'd only be gaining 2db, which is a stil barely a change. Bigger woofers have bigger coils and motors, more effecient, not less. Also, you can try looking for an effecient amp as well. Some of the newer D class are very effecient, a Zed audio Minotaur only pulls like 50amps of current on music and is 1200w rms. You know they don't BS around or lie either when have you use 8gauge power wire!
Anyway, pick the setup based on your goals and budget, not your amplifier's wattage.