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Montana Sky hit the nail on the head--the pyro should be positioned where the pyro manufacturer says. Beyond that, it seems to me that position of the pyro depends on what you are trying to do. If your primary interest is protecting or diagnosing the engine, then it ought to measure before the turbo. Point of fact, aircraft exhaust gas temp probes are placed about 1 to two inches down the exhaust manifold. The best systems have a probe and seperate readouts for each cylinder. Single probes aid in leaning the air/gasolene misture but provides little if any diagnostic help. Multiple probe systems are excellent at helping engine diagnostics. Some even provide graphs to show relative perfomance of each cylinder. For instance, if an injector fails the evidence is immediate. My aircraft has individual readouts for heat temp and exhaust gas temps on each cylinder.
If, however,your first concern is the turbo then post turbo probes are better. Here you can actually monitor turbo temps prior to shut down. True, exhaust gasses have cooled by this time but, the fact that you are only installing a single probe will not provide much in the way of engine diagnostics regardless of where it is positioned. There will be a lag (seconds) in the measured temperature changes but this should hardly represent any risk to anything. I suspect, as Bob said, this is why the commercial fleets use post turbo installs.
Dave
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