Not being a diamond moderator (but having all other privileges), I only have access to the statistics from the past 30 days.
In that time span, though, I can certify that there certainly isn't an excessive amount of spam. I didn't check Sandbox posts (on the basis that they're commonly deleted and we probably want new users to be able to post in the Sandbox); in terms of other Meta posts, there's exactly one post that was deleted in the prior 30 days, from a user who had 101 or less reputation at the time it was posted, that could even plausibly be described as "spam": this one. (For users who can't see deleted posts: it's a direct duplicate of this question, which was closed as such and downvoted to -1; Community deleted it on the basis that it was negatively voted, inactive for 30 days, and had no answers.)
It's worth noting that this is a very small number of unwanted posts, and easily handled. Even then, it only qualifies as "spam" in the sense that it's a post that adds nothing to the conversation on Meta and shouldn't have existed. It's not "spam" in the "red flag" sense of "Exists only to promote a product or service and does not disclose the author's affiliation". (We do get spam in the red flag sense on PPCG very occasionally, but it's nearly always posted to main rather than Meta, and it's typically handled very quickly.)
As such, I'd say that reducing the Meta participation limit hasn't caused any noticeable issues, and the benefit (allowing new users to post in the Sandbox) is fairly notable.