It might be, but the potential for hoovering up a puddle at that height is significantly increased, brad!
And yes, 30mph, sorry - all I can think of is mpg at the moment. My fuel bill is the same size as most peoples mortgage payments: they're really shocked when I tell them!
Dave - nothing wrong with a theory at all, and big props to you for even considering all this.
With the turbo, bear in mind that the radiant heat being transferred between the two halves is comparitively microscopic compared to the conduction through the metal middle of the turbo!
Also don't forget that the compressor side itself actually gets hot due to the compression process....it's generally just easier to intercool the charge afterwards.
I'm mainly interested in keeping the whole back of the engine cooler, irrespective of turbo performance. Cooling down the exhaust housing isn't going to make a significant difference in performance terms compared to allowing the engine to run cooler, allowing less fuel and more timing.
You need to be worrying about having a "perfect" exhaust design (which is, at the very least, a seperate exhaust for each turbo with a link pipe midway) long before exhaust housing cooling becomes an issue. That said, you could always wrap the exhaust side in a turbo blanket to keep the heat in.
I'd rather cool the whole lot, and run higher EGTs, though - you're not trying until you melt an exhaust manifold