It will impact your emissions and you gas mileage to a little extent
and it will get worse over time, although not all that quickly. It
won't effect performance because the PCM doesn't use them when you're
running the car hard.

You could try pulling the one indicated and cleaning it off a bit. You
can't do much because you don't want to force a replacement :) and
because whatever is on there is pretty tough since the sensor gets
pretty damn hot. I wouldn't dip it or spay it, although I know folks
who use brake cleaner and it seems to burn off clean, but you could
brush off anything flakey. I've read of using a propane torch and a
brush, as well.

Dan
2003 Cobra convertible
With some stuff and things