In metal shielding cases, slots, not holes, are the big problem. Slots are long openings and leak on wavelengths up to their length. Small round holes leak well up in the gigahertz range, where high-power noise is less likely.

A high-powered switching power supply, like this guy is building, is the worst case. It's inherently a high-power RF emitter. Switching power supplies are spike generators, so they emit lots of high harmonics. Of course this guy has an EMI problem.

I've run into this with a device I'm building that's powered from a USB port and has a switching power supply. The switcher runs around 300KHz, and I can see noise spikes down to 25ns, which is 40MHz. Keeping it from injecting that back into the USB power end is tough. If the device is powered from a computer, that's likely to cause trouble. USB ports usually have current limit detection, and it's instantaneous current that matters. Exceed 500mA for a microsecond and the port shuts down until power cycled. That's good; port shutdown occurs before the computer crashes due to noise on the power bus. Right now, I'm waiting for some 6.8μH ferrite beads to arrive. 1μH wasn't enough.

The gold standard for EMI testing is to test outdoors in a big, open field far from any RF emitters. Even for some IoT devices, an outdoor test range is used for the final check.[1] Here's one for megawatt-range power gear.[2] For outdoor testing, the unit under test sits on a wooden or Fiberglas turntable, all alone, several hundred feet from anything else. Now you can make far-field measurements. RF anechoic chambers are used for convenience, so you don't have to go out in the boonies for test runs.

[1] http://www.echelon.com/assets/blt2781ab44cb62fe72/PC_BOARD.P... [2] http://incompliancemag.com/article/lessons-learned-from-the-...