As far as I know DVDSubEdit can only change individual timings. I don't recall ever seeing an overarching retime all subs feature.
I tried demuxing, retiming the subs in vobsub, and remuxing, but something weird is going on there. Not only were the subs out of sync, but the audio was as well afterwards.
I'll may give it another try tomorrow, but it sounds like JoeyO is on it.