The sound card tester will work for focal plane shutters and with faster speeds. The tester as built will show you how long the phototransistor is exposed to light, which is the same amount of time that piece of film would be exposed. I'm not up on the modern 1/8000 second shutters, but the traditional focal plane shutters I'm aware of travel at a single constant speed with only the slit width varied for speed adjustment. That's the very reason you need an electronic flash sync speed in which the whole frame is exposed at once. See this page for a shutter tester from a reputable mfgr that relies on this geometry:

In any case the sound card shutter tester will be good for any speed at which the curtain spacing is equal to or larger than the diameter of the phototransistor sensor. Draw a circle on paper to represent the sensor and run two paper curtains over it at varying spacing to demonstrate this for yourself. Even with the sensor somewhat larger than the curtain spacing, you could measure the time between the two points halfway from min to max signal amplitude (halfway up the signal skirts) and get a good reading for shutter speed.