I think the best solution, in terms of shooting time, freedom from color cast, and reduced wear/tear on the shutter (for those whom its a concern) would be a blend between using the app (or just PS), and a 3 stop ND; at least for the typical 3-4 minute type of long exposure I like doing.
Stacking/blending 30 or so 2-3 second exposures avoids the color cast issues I run into with my 9 and 10 stop ND's, I don't need to stop down quite as much either so less issues with diffraction, and I don't need to take several hundred hundreds which would be required without a filter at all.
If you've got an internal timer on your camera, you can set it to do this pretty easily, and while you'd still have to do it in post instead of the app, it does allow a new technique for those who don't have a compatible Sony model.