Open filters are fine on turbo'd engines since if they're running a decent amount of boost, the air around the intake doesn't get time to heat up since the turbo's sucked it up already
There's an article somewhere floating around the web regarding MR2 turbos, and different filters. They did back to back rolling road tests, and also showed how well the filter actually did its job of filtering dust etc out. IIRC the HKS mushroom jobbies were aweful!