One of the charts in TFA shows a discontinuity in admissions rate around 75% UPP. This means that if you send your kid to a HS that is underserved when you enroll, but drops below the critical 75% threshold because too many other families are doing the same, then the school could fall out of the strong-benefit category.

The kid would still have a better chance than if he applied from a high-performing school, but it wouldn't be as much of an advantage.

Socially, I'm guessing the kid could face some challenges because (1) other high performing students might not like him because he's a curve-breaker, (2) teachers would know what the family was up to and could view it as distasteful, and (3) if the student went to UCSD or another school where this is a well-known hack, there could be stigma for having gamed the system/being less-smart.