How much of a boost does sports participation (non-varisty / team captain level) give toward college admissions at competitive schools like Ivy+?
IMO, zip/zero/zilch/nada. For the non-recruit, a sport is just like any other EC. Traits like leadership/teamwork/tenacity, etc can be found in drama club, model UN, and a host of other activities.
It will be regarded as a good EC – nothing more and nothing less.
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If you are ranked among the top in your state and/or you are recruited, it would be a definite plus. Even if you aren’t at that level and you love the sport and enjoy being on the team, do it.
The OP said non-varsity level. This is just an activity. No way to tell if he puts in 30 hours a week or 5. The OP would have to explain more on the application. It’s like putting down ‘band’ or ‘science fair’; without more information, it might not even be considered a ‘good’ EC.
Assuming not even recognition or award at the school level, like any other low level extracurricular participation. Which is not very competitive at super-selective colleges against applicants who have state, national, or international level extracurricular achievement, or athletic achievement high enough to get recruited as athletes.
It won’t.*
But it would hurt if you do no extra curriculars in HS.
So pick ECs that are of interest to you, get involved, and try to find leadership opportunities.
*unless you are looking at Service Academies (e.g., West Point, etc)
Doesn’t matter (although I agree with the exception of military academies noted above). Unless an applicant is a recruited athlete a sport will count the same as any other EC. Do the things that interest and excite you and let your passion come through.