I don't believe the converters speed up the video frame rate from 25fsp to 29.97fps. What happens when converting PAL to NTSC is a pull down conversion. Certain frames are duplicated every second - to make 30 frames out of 25 frames. That's why you get a very jittery motion with cheap converters. So the length of program stays the same because certain frames are duplicated every second. The audio should stay in sync thru out the video
I watch on the fly PAL to NTSC converted television all the time. Watched the Formula One race, which certainly must be shot with PAL cameras. Watch BBC world which must be live PAL cameras yet I'm watching it on my NTSC TV Live and the same length as in PAL countries.
Yeah, its more frames per second or "more frames each second for the same length of program time, no? The frames just go by faster in the same amount of allocated time?
Look up PAL to NTSC pull down and you should get a better explanation then I can give.