I just replaced my Volcano 11 with an SLK-800 with the same fan (Thermaltake Smart Fan II).
I just built my brother almost an identical rig to mine. I gave him the Volcano 11 and his temps are the same as mine at stock speeds.
However, I am now overclocking and my temps are still very reasonable with the SLK-800. I never OC'ed with the Volcano so I can't compare, but I guess that's where the SLK-800 will come out ahead.
I know everyone swears by the SLK-800 and thinks the Volcano is crap, but I really don't see any big difference unless you are going for a huge OC. The Volcano 11 is the same price as the SLK-800 and you already have the fan.