I am having the same exact problem, I sat on the phone with them for 4 hours and nothing, we swapped it out for the new XB6 modem and still the same upload speeds. We then swapped it back to the old modem and I had to wait 5 days for a tech to come out he ran some tests said everything tested fine and reconnected the new XB6 and the problem continued... he didnt have a new modem on his truck so he said I would have to take it to the Comcast store and swap it out for new one. I swappped it and installed it using the Xfi app and still no improvement. I called tech support and they said there was nothing they can do and are sending another tech out. This is it for me.... if they dont have it fixed its bye bye Comcast.
Thank you for visiting the forums! Sorry to hear about your issues.
I've asked a Comcast employee to help you. You should expect a reply in this thread.
You might find the troubleshooting guide helpful.