Checking in for the first time here - just picked up my new iMac Pro (base model) on Monday. Upgraded to 10.13.4 during initial set up, so can't confirm the status 'out of the box', but I'm seeing the boot rom version in the system report (15.3169.0 as others have reported), and also have the same brightness issue. I'm not interested in the auto brightness feature, so have that disabled. Do a fair amount of video work and colour grading etc, so I like the screen brightness to stay where I've set it ... getting the 'return to c.90% brightness' problem after every reboot (I prefer to turn off rather than sleep), which - as others have said - should not be occurring on a computer at this price point, which in part is designed with graphic-intensive work in mind.
Someone also mentioned a Duet bricking? Mine did the same after I (unsuccessfully) tried the SMC reset for the brightness issue. Assume that USB power is also affected by SMC?; happily my Duet was restored after a full disconnect/reconnect. Sorry for the off-topic, but wondered if that might help the earlier poster with the same issue?