I'm building up a TT tandem. Would 175 front cranks & 170 rear make sense for if the pilot is more powerful than the stoker?
Not necessarily. The crank length will be determined by the size of the rider. A powerful stocky person might find a 175mm crank awkward and vice versa, a tall person but less powerful might not like the shorter crank. Crank lenght is determined more by size of the rider. I'm no expert and I'm sure there are more qualified people on the forum to comment.
I'm quite tall and have always used 175 cranks on front of my tandem and 170mm on the back.
bikedynamics.co.uk will provide you all the information you will ever need about crank length
There has been some research into shorter crank lengths improving aerodynamics, sure I heard Chris Boardman say that experiments with cranks as short as 155 have improved performance.
Have also seen that crank length shouldn't be looked at on its own or just as a function of leg and other sizes as gear can be changed to suit.
Essentially you have opened a can of worms!
I believe the aerodynamic gains come from being able to lower the bars more because the knees don't come so high, allowing a lower back angle. Shortening the cranks in isolation might not provide any aero gain. I wonder whether different crank lengths for pilot and stoker has any value for mismatched cadence preference? I have read that shortening the cranks increases a rider's preferred cadence. So, a shorter crankset for the stoker might make sense if the stoker normally prefers a lower cadence. I don't know whether this has been proven!