I would be interested to hear of one.
My first thought was Them, a movie wherein the ants apparently benefitted.
I know of no scientific reports to confirm any cases of this type of benefit--but I would certainly not think it's out the question. One problem is that the great majority of radiation-induced mutations, of those reported so far, are lethal. So it's hard to get data when the experiment keeps wiping out your subjects. My own opinion concerning putative beneficial radiation-induced mutagenesis is that this would be not only in the realm of possibility but also one of probability.