Snubber capacitor VAC handling by frequency of C0G?
Ceramic caps are great at handling spikes; in many applications people unwittingly use them as a second line of defense for ESD.  I wouldn't be uncomfortable using a 500V part for 800VPP (400V maximum instantaneous).  As far as the power dissipation goes, Kemet has a fine tool that will plot the ESR of your capacitor over frequency.  Unfortunately, that tool doesn't cover this series of caps.  However, there's not a huge difference among ceramic caps from competent manufacturers, and Murata has a similar part GRM32B7U3D221JW31 (2000V 220pF 1210) that their fine tool says has an ESR under 0.1 ohms all the way to 499MHz.  I don't think you have anything to worry about.