I don't know if there is difference between SP and MP replay... obviously MP replays usually have more cars so that's the difference I think.
Using 16 bit mode gives few fps more (if you use 16 bit, put on "Dither" option in LFS, otherwise some textures look bad). Also have you noticed that with the so called half-speed trick you can record at very low fps, e.g. 15 or 20, when you double it it's usually smoother than 30 fps normal speed recording.
And many people seem to record with FSAA/AF, obviously on a low end computer you can forget that.
I tried Game Cam some year ago and for me it gave worse FPS than Fraps.
I've heard that too... I have two 7200 rpm drives and haven't noticed any major difference. Might depend on the game.