But thunderbolt speeds are expressed in Mbps while PCI-E speeds are expressed in MBps. So there is a whole magnitude of speed difference. Now if the speeds were actually comparable then I would say this design is genius. Because it would be sized for most people and those who need expansion would attach a box. But given the speed difference it isn't quite there yet.
So as I understand it thunderbolt 1 was PCI-E 2.0 x4. So this is PCI-E 3.0 x4. Now perhaps most of these speciality expansion cards can run at that speed effectively. But somehow I doubt it if they are to accelerate 4k+ full data rate raw video. Plus Thunderbolt has some overhead.
If someone can get the job done on MacBook Pro then this would be great. But this is supposed to be an upgrade for current Mac Pro users who have likely upgraded their machines. It's a whole other level of performance expectation.
To tell you the truth I don't really know much about what video editors need but I'd imagine they need everything they can get. At least some of them. And those people will be pushed to Windows. So this machine by its design is pushing away a subset, however small, of its prior users.
If they haven't left already, I dont see how this would push them away any faster. People are already using Red Rockets and MacBooks. A RR + Sonnet chasis can already do realtime 4K playback and transcoding. Most PCIe 3.0 cards don't even fully utilize the available bandwidth, so it might not be that big of an issue. For most pros, its all about workflow.
Also while I know their revenue model depends on people buying whole new machines it's disheartening to know that not even one (overpriced) option exists for people who want to upgrade or expand a Mac OS running machine. The old Mac Pro premium pretty much priced an upgradability tax. And people accepted that. But now that's not even an option anywhere in the line up.
Apple has been going this way for awhile. I used to hold out every year for the infamous xMac, but it ain't coming through that door anytime soon. Some of the logic for the Mac Pro's expansion capabilities might be to bring it in line with the other Macs. So buy your RR + Sonnet, and then buy the Mac that fits your workflow. Need more raw performance or expandability (via TB of course), go Mac Pro. Need mobility, MBP. Or buy both, and one RR, and when you need to go in the field take your MBP and RR, do the quick edits you need to do in the field, and when you get back in the office do your post on a Mac Pro.
It'll be interesting to see how much it costs. In my mind I was thinking $3k, but I just looked up how much the Mac Pro I have at work retails for, and its over $6.2k.

