The XRackPro2 cabinets are good but they are certainly not the best. The best are sold by Acoustiproducts. The reason I'm able to compare is that Acoustiproducts have a studio where you can go and compare the noise leakage of the different cabinets. They sell the XRackPro2 too, so they are not unfavourable about it, but if you can afford it, they recommend the 'Ucoustic' range. I went along and compared these, and the difference is really marked. So much about noise pollution isn't about the level of db, but it's about tonality and perception. I can't imagine putting either in a recording studio though, it must be a budget recording studio that has server cabs in the monitoring room and not in some purpose built annex somewhere else in the building!
I have had two of the Ucoustic models, one is a passive cabinet, and my lastest is an active cabinet. The great thing about the active cabinet is that you can attach two ducts to it and route this out of your windows (I had my window replaced with glass that had two holes cut out of it so they could be routed straight out, much like air conditioning vents). Of course, if I want free heat, I can simply remove the pipe and let the air circulate in the room, but in summer I'd much rather expel it. Servers do blast out a lot of heat.
The second, third and fourth benefits of the Ucoustic rack are: they are very heavily built and act like a safe to deter theives, they are graded as fireproof or at least meeting industry fire protection standards, and they can be fitted with a dust filter to aid longevity of equipment.