How would we know? It's not very often that someone goes to the doctor because they're too healthy.
The PROPORTION of useful sequences must be small.
Small has no absolute meaning. I think it can't be too small or that random string of amino acids wouldn't have been a useful enzyme.
Mutations generally are harmful.
My understanding is that most mutations are neutral. Of the remainder more reduce fitness than improve it in a well adapted organism, but there are more of the latter than you think.
There is a beneficial effect of low levels of ionizing radiation. Above a threshold there is a linear dose response of damaging effects.