8 ohm sub w 4 ohm amp?

Question one: adding a second 8-ohm sub will let the amp "see" a total 4-ohm load.  So, the amp will put out power at its 4-ohm rating of 200 WRMS (or whatever it is.)  The amp will be performing at it's peak capacity.  Each sub will see exactly HALF of the available power that the amp is producing.  This will be "louder" than one sub by itself by approximately 3 db, or about 1/3 louder to your ears.

Question two: Sony gets off calling it that because, in their tests in their labs under ideal conditions, that amplifier produced a momentary peak output of 1000 watts without self-destructing.  This is why you should never use the peak ratings listed for amps, and always look at the RMS output ratings.  Even these are often exagerated, byt the way, and Sony is not known for being conservative in their ratings.  I would be willing to bet that, tested under real-world conditions with full musical program material, that amp will sustain about 140 to 150 watts RMS per channel within its distortion limits.