The Deccan Traps event spewed out more sulphur, CO2 and toxin than the C-Club crater by a factor of a thousand. And that asteroid was 6 MILES wide. It would have to be an asteriod the size of the planet Mercury to outdo a supervolcano.
You not taking into the account the cataclysmic impact factor, and the resulting energy released.
Look at the lunar surface. There are craters there that are over 180 miles in diameter, some with shockwave, blast zones rays that extend for over 1500 miles across the surface.
Nothing on this planet could equal the energy and resulting devastation by a large asteroid impacting the earths surface.
Another fact is, man has not located or discovered all of the super large asteroid/comet impact sites, due to the fact we have large oceans, and our surface environment constantly is changing.
Sorry, but a large asteroid impact kicks your volcano's butt.