Regarding the bars, the problem is with videos which aren't precisely 4:3 or 15:9. If you're after generating a 320x240 or 400x240 video, then you're going to have to add bars or distort the picture. I prefer to add bars.

Regarding framerate, I've found that the lag between sound and video is smaller when using 15fps rather than 25fps.

EDIT: Aha, just re-read this thread and discovered the "-vo sdl -ao sdl" option - the 25fps movie I generated is smooth and in synch now! Actually, even the full-sized movies are smoother too... will have to re-think this...