ummmm, no, not really accurate to comare a consumer case with a low pwered fan -vs a z8xx series HP machine.
but the proof is in the pudding...
my z820 is currently running all 40 threads at 100% has been for hours now, and the fans are not even spooling up, can't remember the last time i heard the fans, months and months at a minimum.
i have three z8xx machines in rack with five arrays, and two other machines - the arrays are far louder and throw out far more heat than the z8xx series machines they are attached to
you will need to make sure you have 24" of clear cool air front and back as HP suggests, the heat shrouds and related heat managment inside the z series is impressive, and far beyond what you are thinking... not atall an empty consumer case...
the spec we don't care about is the size of the fans, we care about what they do... so the spec's you care about are;
heat dissiaption, 2x 2690's / 2 x k5200 = 1940 Btu/Hr
deskside noise = 26Db with internal hd read / write, 23Db when ideling or using an SSD like we all do in 2017...