Motorola MB8600 and Gig Internet

Recently, Comcast began offering gig internet service at my location. Today, I changed my internet service plan from the Blast 200 Mbps tier (which had been the highest speed offered in my area) to the Gigabit tier. I have my own modem which is a Motorola MB8600 and which is spec'd for Gig internet/DOCSIS 3.1 and advertised as such. However, I have only been able to get at most 350 Mbps DL so far (approximately 10 hours since upgrading internet tiers). I consistently get around 40 Mbps UL so the UL is fine. I looked on the Xfinity My Device web page and my service is now reflecting "up to 1.2 Gbps", but the same page reflects that my MB8600 is only capable of wired download speed of 890 Mbps. Not sure why that is the case, but still I'm not even seeing anything close to 890 Mbps anyway. I've conducted all speed testing on an ethernet connected computer with gig network card. I've tested it connected directly to one of my numerous gig switches, directly to the gig router, and directly to the MB8600 modem. I've used the Xfinity Speedtest site and www.speedtest.net for all testing.

Is it possible that the correct gig configuration file wasn't sent down to my modem even though the correct speed tier appears to be provisioned?

I spoke with a regular customer service rep at Xfinity earlier who said everything looked good on the system and that the gig service was getting to my modem so that the problem must be with my modem. I'm not ready to accept that is the case. How could I go about confirming that the tap out front is actually able to give me the new gig service? I ask this because less than 2 weeks ago I was told by an Xfinity tech here to replace a cut cable that our tap wasn't yet upgraded for the gig service. Not sure if he was correct but Comcast did sell me a 300 Mbps service tier about a year ago that actually wasn't available to my address. Had to revert my plan once they discovered their mistake.