When you measure high speed leaf shutter opening time, you need to think about the effective opening time. For that, I think you need to integrate the amount of light that pass through and not just looking at the opening and closing time.
When calibrating my shutter using a phototransitor and my sound card, I realized that the highest speed are off quite a bit. About 0.20 stop at 125, 0.5 stop at 250 and about a full stop at 500. The other speeds are within 0.10 of a stop. Since I'm not able for now to get the DC component of the signal, I can't validate my assumption. As soon as I can get my hands on a DAQ card, I will check if integrating the signal make a more sense.