The average peak grade is closely below 7b, and 8a is the mode grade (the most frequent individual maximum grade). In contrast to the distribution of all ascents, we do not see a clustering around 7a for the individual top performance. There are similar number of 7a to 7c climbers. There are however much more climbers with an 8a limit than we would expect for a smooth distribution. Keeping this in mind, let us gauge how much more difficult is 9a compared to 8a and 7a: In the 8a.nu database, there are more than 4 times as many climbers who have achieved 7a compared to 8a but there are almost 50 times as many climbers who sent 8a than 9a. Moreover, this is most likely a severe underestimation of the difficulty since I assume that elite climbers are over-represented at 8a.nu compared to other climbers (and top-level ascents which help you to receive publicity and sponsors are recorded more often).
Here, you can also see the distribution of the maximum grade achieved separately for male and female climbers only (85% of all registered climbers in the database are male). Let us start with female climbers:
For female climbers, the average top grade is close to 6c+ and 7a is the most frequently recorded top grade. Male climber's average grade is close to 7a+ and 8a is for them the mode grade. Interestingly, there are much more females who report 6a, 6b or 6c as peak grade compared to 6a+, 6b+ or 6c+ but we do not find this pattern for men.