Is it normal for fans to run at 6,000+ rpm at <20% CPU load?
You need to strip &amp; clean the entire airflow, if that fluff-ball was clogged in the outlet, there's an entire duvet inside. All the machines here get a full strip &amp; clean once a year, with a 'quick blow round' in between. If it's gone 6 years without cleaning, it's likely to be pretty well-insulated in there by now.

iFixit teardowns are second to none - there is far too much for me to try to précis here
iFixit - MacBook Pro 15" Unibody Early 2011 Teardown