The MIRI performance is simply astounding. The combination of large aperture and cold telescope makes it the premier mid-infrared capability, far more sensitive than the previous infrared space missions ISO and SIRTF, or than  hypothetical huge groundbased telescopes (30-50 meters aperture):

A simple comparison is the mapping time required to reach a given area of sky over a given number of picture elements (this is equivalent to but more approachable than the metric "astronomical capability" introduced by the Bahcall decadal survey to dramatize the potential of SIRTF).The gain of JWST is shown below. It maps more than 10,000 times faster than even SIRTF or a hypothetical 30-meter aperture groundbased telescope.

This gain is the key to the immense potential to make new discoveries with the MIRI - the astronomical community will use it to probe the Universe to unparalleled depths.