For future reference:
If you're using the replay rewrite script and getting extremely fast replays, do this:
Edit the script.
Find this line (roughly line 977): unsafeWindow.nextFrame = function() {
Shortly after it you'll find this line: unsafeWindow.currentFrame++;
Change it to this: unsafeWindow.currentFrame += 0.5;
That should fix the framerate problem.
if you don't care about losing any edits you've made to the script, I've already put the change here:
http://userscripts.org/scripts/show/31640*edit:
I've updated the script again to account for Bort's selectable framerate, so if you update the script, this should no longer be necessary.