Is there any impact whatsoever of enabling FILESTREAM on active SQL Server?
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