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.
