A common way to run lots of boost on turbocharged cars is to use 87-93 normally, and E85 under high boost. You get the high octane when you need it, without the 30% hit to fuel economy when you don't.
I've seen this done before, but I wouldn't consider it common. In my experience, the two most common approaches for mitigating knock on high boost street cars involve either running E85 all the time (or a reduced mixture like E30), or using a water/methanol injection system. A secondary fuel system with race gas is more common on nitrous cars though.
Anecdote: I'm running a water/alcohol injection system on my 335i (tuned on higher than stock boost). It really helps on the hotter days. Currently, I'm running a mixture of ethanol (everclear) and distilled water. Most people use methanol, but it's not really compatible with the viton gaskets inside of my pump.
