It's persistent - though there's a bug in the current build where subsequent gangs won't get the health bonus when recruited. That is, when you get Strangefellows 2, all existing Thugs and Gangs get +1 health (and +1 to their max health as well), newly-recruited Thugs start with 2 health rather than 1, but newly-recruited Gangs do _not_ get the additional health that they should be. This will be fixed in the next build, which will come out later this week, once it's gone through QA!