As @skieurope said, there is no admissions ‘cost’.
Like any review, you have to factor in what you know about you/your son, what you know about the objective specifics, and what you know about the reviewers. Some of my favorite classes have been the hardest. For some of the Collegekids, a 70 page research paper on a subject of their choosing was the stuff of dreams- but for others the stuff of nightmares. Objectively, how the assigned amount of work fits with the assigned amount of work in other subjects will matter.
AP Seminar / Research is the most heavily school/teacher dependent AP of them all (followed by USHx & Bio, ime -but your experience may vary).
Not your question, but help him keep the definition of ‘top tier’ as broad as possible, and to explore the math options at colleges that aren’t the usual suspects. Also, if he is a serious math guy, consider looking outside the US (where the only APs that will matter will be the quantitative ones). If you/your son decide to explore that route, look at @AnnaW’s ‘math head’ and ‘math head update’ threads.