Rackmounts have pretty significant airflow and fans, and sort of focus on airflow over quietness. The ones I used to own sounded like a jet plane. Gamer cases are designed for cool running - they have larger fans with slower speeds - servers have LOTS of high speed fans. Your case is probably the same size of a 2-3 U server
There's 3 fans blowing air RIGHT over the processor and ram, at high speeds (to hell with noise levels). The servers likely also run in lower ambient temperatures in the conventional server room - the case I just linked has a upper operating temperature of 35 degrees.
The cosmos has a big case fan (for everything) a vent fan on top, and totally the wrong configuration for the job. The focus is on overall airflow, since you also need to cool the graphics card and stuff.
I'd suggest going for the same sort of heatsink you'd go with for an equivalent enthusiast level processor if you wanted to be safe - in this case LGA2011. These may still be passive, but are designed for cases that don't sound like a small airport.They're designed for the more likely environments a 'tower' style case will be found in - offices and such, with people, not computer centric ambient temperature
Another thing to look at is the TDP of the two processors. Your Xeon has a TDP of 95W, the top of the line sandy bridge E (which shares a socket) has a TDP of 150W - so a SB-E heatsink SHOULD be able to handle things under normal conditions