Apparently there is a new
eficheck routine that runs on macOS High Sierra that detects if your Mac is consistent with a database of known good firmware signatures. Though it appears to just report a warning with the offer to send the information to Apple, it does raise concerns about whether or not a MacPro3,1 or MacPro4,1 are going to be prevented from running High Sierra as a result.
Yes, I'm aware that a MacPro4,1 can be flashed to 5,1 to make it officially supported. For the 3,1 I've read that so far it will work as long as you have done the Bluetooth and WiFi upgrade and hacked the installer's plist to recognize the 3,1's motherboard. While the 5,1 might pass, I'm imagining that the 3,1's firmware would definitely be absent from a known list of good images.
Is this a blocker or just an annoyance?