1200w Amp good for 400rms sub?

The more experienced you are, the more you can deviate from matching power output of the amp to RMS of the speaker.  With experience, one might power a sub with twice the output as RMS....on the other hand, someone else might power the sub with one half the output.  But as a guiding rule, you should closely match the true power output of an amp to the true RMS of the driver.  Take into consideration, of course, that the power output of the amp is dependent on the driver's impedance.  Now, some amps are overrated, and some subs are overrated....but the sub is more than likely rated closer to true than the amp might be.  A Sony has a tendency to be a braggart, so overrating is very possible.  The Kicker is most likely rated true.  If you have the Kicker wired to place a 2-ohm load on the amp, you have to look at the amp rating at 2 ohms.  That's the matchup you'll be looking for.

The best way to make sure you don't send too much power to a driver is to use a multimeter, test tones and knowledge of Ohm's Law to determine the output voltage the amp should be sending.

The "10-15%" you heard should be forgotten.