Is there any impact whatsoever of enabling FILESTREAM on active SQL Server? I may have to enable Filestream on a live SQL Server at work, but first need to know about the possible impact/implications of doing so on the performance/operation of the databases held therein.

I have done some reading and have not found anything daunting, but since it's fairly important, thought I had better check on here.
Enabling filestream by it self will not have any impact on the databases not using filestream. Queries on filestream data will be done using system cache not buffer cache so you might need to make sure that the SQL server service is not using all availible resources, see http://www.databasejournal.com/features/mssql/filestream-and-filetable-in-sql-server-2012.html