WestCoastGuy, consider the shooting of fireworks with a 30s shutter speed. DFS-in-camera greatly extends the time waiting between shots. The way to reduce this delay is to turn DFS off in camera, take just 1 shot with the lens cap on for 30s for a 30sec-dark-reference-frame and then subtract this in PP. The dark frame should be taken in the same session so the ambient temperature is the same. Since some sensor self-heating will occur, particularly in Sony Exmor sensors used in the K-3 & K-5, where thousands of ADCs are included around the edges of the sensor, I'd take the dark frame at the end of the session rather that at the very beginning.
https://en.wikipedia.org/wiki/Dark-frame_subtraction
The freeware raw converter, RawTherapee, has such an option:
http://50.87.144.65/~rt/w/index.php?title=Dark_Frame
If in a session you were taking both 15min & 30min exposures, then you would need to record two different dark frames.
Dan.