Using Esperance DV ram disc utility - i can get 14 layers of dv movies to playback at 25fps on my Intel 3.0Ghz 8Core mac...
http://www.mparrot.net/index.php?page=freewares&lang=en
using a seagate 15k5 scsi disc gives about 7 layers with same footage.
You can put stuff on a RAM disc and on SCSI at the same time and get improvements...