We did something similar at Facebook for iOS and OSX automated testing and a few of them doing iOS app builds.
Here is a post that Jay Parikh (VP of Infrastructure) made about it. http://tinyurl.com/cnvss4v
Our density isn't as high (we have 64 minis) because of cooling and cabling that we designed according to our datacenter cooling standards.
@jurre - If you want to chat about our design, message me and I can put you in touch with our hardware designer.