According to this querythis query, of the 259 questions on Meta with a score of -20 or less, 145 questions hit the proposed rep cap. 66 of these were closed (for various reasons). Of those 145 questions, 75 represent a negative rep change of 50 or more (37 of which are closed), and 44 represent a negative rep change of 60 or more (19 of which are closed). The average rep "saved" on the 145 questions (that is, the absolute number of points after adding 40) is about 16-17. I did not take account deletion into consideration when calculating these numbers. I have not looked into all of these questions to their relative value to the community.
replaced http://data.stackexchange.com/ with https://data.stackexchange.com/
replaced http://meta.stackexchange.com/ with https://meta.stackexchange.com/
replaced http://meta.stackoverflow.com/ with https://meta.stackoverflow.com/