Performance impact of sp_configure blocked process threshold change
Minimal hit. The cost of evaluating the blocked process list is already there since this is how deadlocks are detected. The cost of generating and writing an event in the server side log is quite low and any server IO should be able to handle an extra write into LOG/*.trc every 5 seconds (At worst). But 5 is a quite low setting, you may be flooded by reports. Is your server server blocking so well behaved as to consider any 5 second block an incident to investigate?

Make sure you monitor the generated traces and make sure you don't cause an self-induced outage due to disk space exhaustion...