it will work just fine.the subs 800w peak means thats the max the sub can handle,not what it needs to run.look for the mimimum wattage specs.that is the minimum wattage it takes for the sub to run efficiently.
Depends on the OHM rating of the speaker.
The amp is rated for 425 watts @ 4 OHM and 700 watts @ 2 OHM.
Running a 6 or 8 OHM speaker will cause it to overheat. or placing it in a restricted area without good air flow will cause it to overheat.