iPhone 8/X can achieve up to 500Mpbs download speeds on Premium 4G with 3CC CA (but only in central Japan for now). It’s the same for story for KDDI and SoftBank. An interesting side note is that the Apple Japan specs page originally published the iPhone 8/X top download speed as 800Mpbs then changed it.
5 months later Docomo has yet to upgrade their Tokyo area 4G network iPhone speed to 500Mpbs. The Docomo Speedtest app always shows Tokyo @ 450Mpbs (purple) with 500Mpbs (green) just a month out. But this Docomo tease has been going on for a few months now, the Green Speed is always just around the corner.
iOS 11.3 beta 5 bumped the Docomo carrier file number to 32.0 so maybe, just maybe the Green Speed finally arrives with iOS 11.3.