Join Date: Mar 2006
Location: Vancouver, Canada
Bikes: Rodriguez Shiftless street fixie with S&S couplers, Kuwahara tandem, Trek carbon, Dolan track
I would think that you'd be better off using two different crank lengths, say a 165mm on the short leg side coupled with a 180mm on the long side. This reduces the leg length discrepancy to 2mm, which can be easily shimmed. Both crank lengths are readily available.
If you were to use only shims, 17mm in shims would be extremely unstable, as most pedals require part of the shoe to rest on the pedal for stability. If you used mtb SPD cleats, you could use a road shoe and get the SPD road adaptor, which would provide an extra non-cleat mating surface to the pedal. If you shimmed out a Look or SPD-SL cleat, you could end up with the sole slightly above the pedal surface.
Also, note the circle described by your foot using different length cranks vs. shims. With different crank lengths, both L and R pedal arcs would be centered on the bottom bracket spindle. With shims, the pedal arc on the short side would be slightly higher than that on the long side. The center of each circle would be different, with the shorter side having a higher center.
Just some things to consider from a non-professional perspective, so take it all with a grain of salt.