Would be very useful, the problem with collecting shaft speed is that the instrumentation isn't cheap and isn't terriably reliable in the field. Backpressure is alot eiaser as they do make high tempature transducers.
I log turbo speed with my PRM Data logger, and it has worked flawless for two years. I busted the sensor at the end of this year so I need to replace just the sensor and it is 85 bucks, no priced to bad. I don't remember the price for the feature but hook up was easy. Reed at Supernatural Turbo put the sensor on the turbo for me.