I dont think that matters.

the track/sector being read from the optical disk will be stored in the buffer at 1.3MB/s or whatever speed the DVD drive is. the SCSI bus/Mac will read it from the buffer at what speed it wants to. 500kB/s. Then the request for the next sector comes in. etc...

the drives operation speed isnt affected by the SCSI readback speed. at least I never known it to. i remember running 48X and 52X CDROM drives on ooooold systems that had nowhere near the speed to read them at that speed. but it would read anyway.