var homePageId = Config.Get<PagesConfig>().HomePageId;
var homeId = Config.Get<ProjectConfig>().DefaultSite.HomePageId;
Just whipped this up on our new 5.2 dev site. Note it's not a multi-site and I don't know how that behaves.
Both values returned the same ID (I'm assuming it's the real home page ID!)... Anyway, the 2nd one doesn't say it's deprecated so I'd go with that.
Oh yeah, the 1st one might have returned a value, rather than Guid.Empty because this WAS a 5.1 project, upgraded to 5.2...
Hope that works for you!