Hello jplw

                 T25 is can support Up to 949Mbps speed so it is capable of providing 100Mbps speed. But if your looking for 100Mbps upload speed you need to help us with your current speed plan. 1Gbps speed plan will provide up to 35Mbps upload speed so it depends on the speed plan you are in.

My Xfinity plan is 800 down, 100 up. I get only about 23 up.

As you mentioned speed plan is 800mbps and receiving only 100mbps, make sure that you are checking the speed test through direct connection since its cable modem.

If you are getting slow speed internet through hardwired connection, I suggest you to check the cable signals.

To check the cable signal levels, follow the steps in the given link:

http://arris.force.com/consumers/articles/General_FAQs/T25-Cable-Signal-Levels

Thanks. I had tested directly to the modem with ethernet, and was getting an upload speed of about 22.5.  My upstream power levels are too low (32.5), but so are all 4 levels in the picture in your example...is there an error in that page?  How would Xfinity boost the upstream level?  They claim everything is within spec.

Since your upstream signals are too low, Contact your internet service provider and ask them to adjust the signals within a range.

If they refused, then ask them to do Re provisioning the modem by deactivate the mac address and reactivate the same.

After that if the problem persists, Please visit arris.com/selfhelp for more information.