What are the SEO benefits/risks of converting unique URLs to subdomains of our primary parent company?

The SEO benefits could be having incoming links to a single primary domain name and overall IT management may be easier. A potential downside is that any specialty domains you could lose some of their search specificity when grouped with the larger parent domain. If you merge the sites then you need to set up 301 Redirects so search engines will understand that he page has moved and visitors will be instantly shown the content they expected. 

Last year Google announced that a page's loading speed is now a factor in search result ranking. 

If the websites remain on their own domains then you have the ability to use that domain's subdomain space to accelerate page load time by allowing more simultaneous connections to the web server. For instance, using http://images.crchealth.com andhttp://javascript.crchealth.com to serve up all of your graphics and javascript can make noticeable performance improvements. 

Generally a browser can only have two open connections to a web server at a time. That means you can get two files from www.crchealth.com, two from images.crchealth.com and two from javascript.crchealth.com. If you queue them up all on www.crchealth.comor substanceabuse.crchealth.com it can take three times as long as simultaneous subdomain connection. You could also store static files like images, scripts and CSS on a content delivery network (CDN) like Amazon's S3 or Rackspace Cloud. 

A great option is a single content management system (CMS) that can host many websites on the same server so you do not need multiple databases to run the many sites under your umbrella.