Thing is, what's the one item that destroys the street-ability of a high performance engine? A long-duration cam.... and they are a necessary evil on a N-A engine for decent performance; ESPECIALLY so in a slant six because the head is so crappy.
A turbo engine has no use for a long duration cam, nor high revs... they have a personality like a stock motor until they get into boost... a perfect setup, if you limit your boost to 10 pounds or under, and keep the spark advance under 18 degrees. It's a pump gas motor.... and is still pretty quick.