A great deal of a PC's heat exchange happens through the case. Plastic shells are therefore not a good idea. (If you don't believe me, wrap your machine in a towel, leaving the front and back open, and watch the temperature go up.) And this one has less surface area. My guess is it will actually run hotter than the same equipment in a standard case.
As to Dell's engineering for temperature mitigation -- a few years ago someone gift me a top-of-the-line Dell that had a chronic overheating issue. It had the hood-and-distant-fan arrangement that OEMs seem to like, but no CPU fan and only the most minimal heatsink, like we mighta used on a 486. I removed the hood and the crappy heatsink, added a standard CPU heatsink/fan (nothing special, just a cheap stock model) and the machine's operating temperature dropped by 40F degrees (yes, FORTY degrees Fahrenheit).
So much for all the engineering that's supposed to enhance cooling, eh? This was when I concluded that, given that excess heat kills a machine in about 3 years, these damn things are *designed to die*.