Let’s establish scope. If the mutation occurs after conception it affects only that one cell and whatever other cells descend from it. In that case, pain is hard to imagine in the general sense.
If the mutation occurs in the egg or sperm prior to fertilization then it can have full effect. There is, for example, a genetic condition which makes pain a non-event. People with this condition wind up not noticing major damage – a broken leg doesn’t hurt, but it messes up your walk a lot.
Going from there back to ‘normal’ is the real blessing, since pain is a great teacher. That’s not a whimsical statement; the world is full of harms, most of which are easy to avoid. Pain teaches you what those are.
On the other hand there 24,000 – ish total genes in each human cell. They get used and reused in baffling combinations. Superficially, mess up one and you have 23,999 other good ones to prop you up. But of course the interdependencies among our genes are overwhelming. There is just no way at present to write a three-page paper 24,000 times to support a study into what might result from a given one of the many possible changes per gene, times 24,000.
And of course a relative handful of the supposed quarter-million to half-million “gene changes” – these numbers are completely made up – will actually improve you. As of now we are like the infinite number of monkeys smacking typewriter keys – one of them WILL write faithful copies of all of Shakespeare! – in our understanding of this fabulous typewriter called DNA.
Oh, and Another Thing 