Just checked in with this, and would like more linkage between SLA options and companies; while you can set the company SLA via API, this doesn't actually add it to the list of companies in a particular SLA. And there's no API to manage SLAs.
When adding 800+ organizations, this makes SLA management basically impossible; there's no way to assign, say, half of them to SLA programattically or easily in the existing GUI interface.
This API endpoint allows you to update an existing SLA policy with the list of companies to which it applies to. The companies can be passed as array of values (Company IDs) using the applicable_to parameter.