I don't think there is any way to answer this question without knowing what specific gun is at issue. Nor can an answer be very serious without understanding what is meant by "sport" use. If one is thinking of playing actual games, then the rules of the particular game will have a lot of influence over whether certain modifications are permitted and/or what competitive consequences they will have.
Regarding the two specific suggestions:
Flashlights: Flashlights on guns for civilians are great if you plan on identifying potential targets by covering them with the muzzle. IOW, they're not as great an idea as lots of people seem to think. I am fortunate enough to live in a time and place in human history where my dwelling is equipped with electric lighting. Similarly, very good flashlights can be had for much less than $100 that fit easily in a pocket (even a suit pocket or the coin pocket of jeans) and can be carried all the time. If I'm trying to decide whether a dark blob is a threat or a family member, I'm going to make that decision before I point a gun at them.
Probably the best thing to do with a "weapon light" is to do what Bob Vogel (a long time cop who is a top-tier competitive practical shooter) does: take out the bulb and fill the thing with lead to make it into a weight. Using one of those, a polymer pistol can start to be as flat shooting as a steel-framed gun (albeit just as much of a pain to carry, if that's on the table).
Dot sights: Dot sights are definitely an interesting technology that does have the potential to improve shooting performance. They'll probably never be as absolutely fail-safe as fixed iron sights, but it seems like they can get into the reliability range of adjustable iron sights. The biggest issue with them is that acquiring the dot (on the draw or after "dismounting" the gun to move) is not something done quickly without practice. It's both painful and amusing watching shooters who are at least decent shooters with iron sights come into competition with dot sights for the first time - and start tracing figure-8's in the air with their gun as they desperately try to find the dot. It takes time to build an "index" that consistently gets the dot to appear in the window.
Of course, it takes time to build an index with iron sights, too, and lots of casual shooters haven't developed that, either.... but if you've already got a decent index for iron sights, it will take a few reps (like maybe a few thousand draws) to get back to where you were. Once there, the red-dot starts to become a gigantic advantage.
Once again, though, if "sport" use includes competition, the user needs to be aware of the rules-based effect that putting a dot on a gun will have.