I have the same exact issue where. I consistently get a speed test of 94 Mbps when I have a 300 Mbps plan. I've tried both SB6183 and MB7621 modems, which are both Docsis 3.0, and they both exhibit the same issue. I'm running these tests with a direct ethernet connection to the modem using a fast ethernet adapter.
Similar to the original poster, I was previously able to get the expected speed using my same SB6183 modem. I think that Comcast must somehow be erroneously capping the speed under 100 Mbps for at least some of us that that own our own modems. I have had numerous calls with Comcast tech support regarding this issue, and they always say that the signal is fine and that there must be a problem with my cable modem.
I actually had this same problem a couple of years ago, but the problem eventually just resolved itself. Back then I tried upgrading to a Docsis 3.1 modem, but that didn't resolve the issue, so I highly doubt that upgrading to a Docsis 3.1 modem now would fix the issue now.
We are having the same issues with our MB7621. Comcast tech came out and said it was our modem as well. We are consistently getting speeds of 95MBPS download an 15 MBPS upload when we are paying for up to 600 MBPS.