In my case Comcast is 800/20 or 1000/35, but they refuse to even advertise the upload rate so you don't know when you're signing up. There's a somewhat outdated, very hard to find page that lists these [1] but many tiers (including mine) don't seem to be shown.

[1] https://www.xfinity.com/networkmanagement

ISTR, and this was ~3+ years ago now, that Xfinity/Comcast is quite hard to even find out what the upload speed of the different offerings is. I even was chatting with their support and asking and they were cagey, like "why do you want to know?"

Currently, at least in my area, after you've purchased a plan (potentially with a term agreement), you can log onto the Xfinity website and see what your expected upload speed is. Mine says 800 / 20, although I think it's possible that the 800 is really 900 and they undersell download speeds to avoid customer complaints. I regularly max out a Gbit line in iperf tests to nearby servers.

I wonder what at upload speed will the acks be congested and prevent full download speeds from occurring.

Is that even possible with SACK?