Once more... I'm pretty sure that once you exceed the amount set in that field, you are NO longer confined to spam checks, specifically when dealing with messages, since AskiMet is part of the spam detection process.
The only area that I'd be unsure of is where it checks other areas for user input.... like this
If you are getting more than that... then yes, there may be an issue there. My site is not that busy... but it's only had about 120 API calls over the last 3 months, and that's WELL within the number of posts (albeit slightly high for new vs existing users but I do use an RSS feed that may come into play).