Daisy chain Thunderbolt to FireWire 800 to FireWire 400

Yes - I have routinely used up to 4 self powered devices one one FireWire port.

Tom's hardware report that at idle most laptop sized spinning hard drives use less than 1.5W idle and less than 4W at peak - so firewire should start to run out of power between 8 to 10 drives chained together assuming your mac is putting out at least 80% of the design power and they don't all spin up at exactly the same time.

It depends primarily on how the devices are connected.

Say we have three devices connected:

A(400) <--> B(800) <--> C(800)

Firewire is processed in chips. 800 speed firewire chipsets will pass 400 speed along the 800 bus at 400 speeds as data is passed along the chain. So anything C sends will be seen by B and A as 400 speed, and it will exist as 400 speed on the 800 bus between A and B. It doesn't increase the 400's total bandwidth, but it does take twice as long to transmit along the 800 bus as it would if B had sent it natively.

When B sends to A it will be at 800 speeds.

The problem is that if A is particularly communicative then it will drag down the total bandwidth of the 800 bus between B and C. If A transmits at full speed, the 800 bus becomes a 400 bus. If A transmits 25% of the time, then its 100mbps data consumes 200mbps of time on the 800 bus.

When C sends data to A, it will be at 400 speeds, and use up twice the bandwidth on the 800 bus than it would if it were transmitting native 800.

This is called multi-speed concatenation.

When B notices C sending 400 speed data, it simply passes that data along the 800 bus at 400 speeds. When that packet is done it can immediately send the next packet at 800. The transition is instantaneous, so there's no additional bandwidth lost, but again, if A talks a lot, it can reduce the total bandwidth on the 800 bus significantly.

B(800) <--> A(400) <--> C(800)

This configuration will reduce the total bandwidth through the entire bus to 400, regardless of the chipset used in each device.