There are disorders caused by an organ or brain area being hyperactive; in theory I suppose a tumor that was damaging/squashing such a thing could be of short term benefit.
Inspired partly by an incident mentioned by Oliver Sacks in one of his books as I recall; about a woman with an imperfectly treated case of syphilis who put off treating it for some time when it started causing her brain damage because she liked how it was lowering her inhibitions. And partly by my father, whose hyperthyroidism was treated by destroying part of his thyroid with radioactive iodine so the resulting hyperactive tissue was producing just enough hormones and not too much.
People with Williams Syndrome are often (way more than the general population, anyway) musical prodigies. They also tend to have excellent social skills.
This is a tough one, but I’ll play devil’s advocate. Lets say a man has a tumor (non-cancer) that affects his hormones in a way that cuts his testosterone output. Slowing of the sex drive and other typical male behaviors might help him complete his life’s work with less distraction.
Or lets say there’s a comedian who has a tumor that is affecting his thyroid and he develops minor hyperthyroidism. He’s more amped up and a faster thinker, which produces more jokes compared to his normal peers. Or if this person is an athlete, a higher metabolism could help them burn fat or outperform their normal peers.
Of course, there are major downsides here. Low testosterone can lead to a lack of a sex drive, ED, fatigue, etc. Hyperthyroidism can lead to long term organ damage.
Much loved British politician, the late Mo Mowlam, was instrumental in Northern Ireland in breaking ground between deadlocked parties, that eventually led to the Good Friday Agreement and the ceasefires.
Mowlam was suffering a brain tumor at the time she was appointed to government, a fact that she concealed from her colleagues.
Her approach to the negotiations was extremely personal and personable, and she behaved in an unorthodox and “unprofessional” manner - something that broke the ice with many of the negotiating parties.
There has recently been a Channel 4 drama about her life that suggested that her entire approach to the Northern Ireland negotiations was based on behavioral changes (in particular disinhibition and lack of concern for the consequences of her actions - such as whipping off her chemo wig during negotiations) caused by the tumor; her success would not have existed without her illness. Not sure how much credence there is in it.
I read once that people suffering from cystic fibrosis were unusually resistant to one of those big plagues (can’t recall which) that swept through Europe a few centuries ago, primarily because the the symptoms of CF were essentially the opposite of the plague’s symptoms*, and so canceled them out, or at least diminished them enough that the person could survive the plague.
*IIRC, it was that the plague broke down cell walls, while CF makes cell walls unnaturally rigid, or something like that.