Hmm… according to some quick research, as long as the bus is operating in bilingual mode, you should have maximum throughput from both devices (ie, the bus doesn’t slow down to FW400 speed because you have a FW400 device connected as long as you’re not daisy-chaining through a FW400 device.
Not sure the mac Pro has multiple controllers - it’s quite rare to see a system with multiple separate FW controllers unless they’re add-in cards.
Just as NeilR points out, my Gen2 Drobo is “bursty” on transfers. So you may be seeing some slowdown based on FW devices “clogging the pipe” for short bits. We see this happen in the video world (where I work) and it applies to audio as well - since real-time data transfer (low-latency) is critical, it’s better to have a slower, constant data flow, compared to intermittent bursts. While the average transfer rate may be the same in both cases, the bursts cause latency that can mess things up.
Say you’re in an underwater chamber. You need to exchange 7.5 liters of oxygen per air. The air system on the surface sucks out the stale air and pumps new air down. If the system provides a constant flow of 7/5 liters/minute, it’s easy to breathe, no problem. But if the system exchanges 37.5 liters of air every 5 minutes, you’ll likely die (or at least pass out) before the next inrush. Probably not a scientifically-accurate example, but you get the point.
In the case of data-flow, “bursts” from one device can interrupt the nice steady flow from another, and a “stall” even if it’s made-up-for by higher data throughput later, can be problematic.
Here at my office we’ve edited medium-bandwidth video from our DroboPro. But, that’s a DroboPro, not a Drobo Gen1/Gen2, and our editing software employs a cache that lets it “ride through” short interruptions.
If your software has some kind of pre-caching/buffering function that may help, but I still agree with NeilR.
At the very least, your current Drobo would make a very nice off-site/lock-away backup. If I had 99 Drobos (I think there was a recent contest/request about this on the Drobo site), a number of them would serve that purpose, littering my associates with my backups, haha. At least it’d give me reason to get out of the house and visit people regularly.