I have a similar issue. I purchased the 175 Mbps package about a month ago, and have never broken 98 Mbps - wired or wireless.
I upgraded my Modem to an Arris Surfboard SB6183, a DOCSIS 3.0 cable modem capable of Mbps.
I alos upgraded my wireless router to a Netgear R6080 AC1000, capable of 300 Mbps + 700 Mbps.
Lastly, I upgraded my cable from the modem to the router with a CAT6, so it can handle speeds above 100Mbps.
Still doesn't seem to go any faster than 98Mbps (at best and is often more like 40-80 Mbps).
Appreciate any help.
You could try calling 1-800-comcast and ask a rep to completely remove the modem from your account and to "re-provision" it from scratch. Also ask them to confirm that the provisioning and account data in their database is correct and complete and that the correct configuration / bootfile for your subscribed to speed tier is being pushed to the modem.