When you daisy chain, say, two FW800 drives on a FW800 bus, the speed of the FW bus doesn't get reduced. The speed of the FW bus remains constant.
Say you have two FW800 drives in a daisy chain: a project drive (with audio streaming) and a sample drive (sample streaming). The variable here is how much data is being read off of the drives at any given time, not the speed of the bus. And the amount of data being read off of either drive at any given moment will not likely be equal or balanced.
You can connect up to (I believe) 127 FW800 drives in a daisy chain and you won't impact on the speed of the bus itself.
Now... any hard drive (FW, eSATA, USB) has a limit to how much data they can deliver at any given moment. But let's say you had this really intense production where you were streaming the maximum amount of data possible off of those two FW800 hard drives. Would the combined amount of data from both drives bring the FW bus to its knees? That's one question I can't answer, but my experience tells me that it's possible to daisy chain multiple drives and play back an intense amount of data (movie scoring, requiring audio, video, and samples) and not encounter "disk too slow" error messages.
I think that would be an interesting question to investigate, tho. Meanwhile, the main point I'm trying to make is that connecting multiple drives of like speed on the same-speed bus doesn't not reduce the speed of the bus.
Now, daisy chaining FW devices of mixed speeds is another matter. FW400 and FW800 devices connected on a FW800 bus need to be configured in a particular way in order for the FW800 device to communicate at its full speed. That's a subject for another post which, if I have time, I'll write later this evening.