a single post can have a huge impact on the reputation without any big
This can at times be true, but usually the well upvoted posts are either very informative and well written, so arguably deserve the rep, or were written a long time ago and have gathered many votes over time.
A lot of rep earned from either of these scenarios is fair, because that's how the site works.
Most users with high rep have gained a lot of their rep over numerous other answers and activity.
So, even without their having a highly upvoted post or two, their other moderately well upvoted posts would have earned them the same privileges anyway - there's only so many privs you can earn.
100 accepted answers are much more valuable than one answer that has
been up voted 300 times.
The accepted is primarily valuable to the OP, as it signifies they got their answer. While this can also signify it is useful to others because they have the same question, it's not simply the case where the answer which helped the OP also helped everyone else.
I have seen posts with accepted answer at the top of the answer pile, with a score of 4 or 5, and answers below it with a score of 20, or 30 or more.
Given the site is about more than one person getting an answer (the OP), the most upvoted ones are more valuable to the community and others users - as reflected in the many votes it gathered.
An answer being accepted has no bearing on this, as accepted is only one person's opinion, where votes gathered are everyone's opinions.
There exist many examples, but let me pick just one example here: this
Checking if an element is hidden
What do you mean precisely by "many" examples?
Even you had found 500 examples (which I very much doubt) consider that Stack Overflow has 16 million answers, even 500 examples wouldn't really signify a serious problem.
Also, while your example could be argued as receiving too many votes given the answer content, and perhaps argued it should be capped, how do we decide who's answers should stop getting rep? And who decides this?
Would you be happy if the answer in your example was your answer and at say 100 votes you rep was frozen?
Wouldn't it be more even-handed to limit posts to lets say 10 or 20?
Even if a limit was to be implemented, 20 is simply way too low.
Heck, 1,000 can and would be argued as far too low by many users.
With such a ridiculously low rep/vote cap you will greatly reduce users from writing "great/amazing" answers.
So what if many do it for rep as well as helping? If they spend time (20/30 mins, an hour) to write a great/fantastic answer, even if they did it mostly to get rep, it still provides a fantastic and very useful answer to hundreds/thousands of users.
It is limited already
There is already a limitation, the rep cap.
Which stops people getting too much rep each day which arguably limits their reaching privileges too quickly.
The rep cap seems to control the rep earned to a sensible balance of:
time on site + experience vs rep and privileges earned.