Originally posted by TomB_tx A different flash will not let you use faster shutter speeds with it.
It would make the process easier and more repeatable by using a flash that has manual control. For things like this, fighting with P-TTL will just lead to headaches.
I've done something similar to this with my YN-560 in a very dark room and used a 10 second exposure. That was with my K100D that had a pitiful burst rate, so that was out of the question. I triggered the camera, triggered the event I wanted to capture, then manually popped the flash by hand at the moment I wanted to capture. It definitely wasn't easy, but it worked well with that slow old camera. If I were doing it again with the K-5 I would just leave it on burst and 1/180s, at low power the Yongnuo has no problem keeping up with the K-5.