It's actually pretty common from what I see. It it an anti-surge ported shroud. When the turbo spools quick and is trying to flow more air than the engine can handle (or when you lift abruptly and the blowoff valve can't flow enough) it can pile up and burp back out the compressor, causing it to stop spinning and being rather rough on the bearings. To combat this they drill the holes. When the turbo is near the surge region the pressures are such that the extra flow basically recirculates around the compressor wheel, from the volute to the intake and round and round. This lets the compressor keep spinning smoothly. When the engine sucks more air the ports suck air in.
I realize this is a forum link, but the guy copied and pasted an SAE article basically. Some good pictures there: http://www.turbobuick.com/forums/threads/turbo-compressor-maps-effect-of-ported-shrouds-and-the-tornado.381448/
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1971 911S, 2.7RS spec MFI engine, suspension mods, lightened
Suspension by Rebel Racing, Serviced by TLG Auto, Brakes by PMB Performance
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