It's not a cap on speeds per se. Your lines just can't physically support the full upload speed. Odds are if they tweak the upstream back to 3200kbps and then it drops again, that means the line isn't fully stable with it there and the system automatically adjusts the line to a more stable configuration.
Also are you sure you're on 50/10? If you have Optik currently, I can't see Telus giving 50/10 simply because both the TV and the internet could/would bottleneck so bad with the attainable speeds you've got. What do you see when you run a speedtest? My line 1 & 2 levels look similar with the exception of my actual upstream line rate being a bit faster (4140 Kbps on each line) and I'm limited to 25/5, with Optik TV.
At this point Telus isn't investing any more in copper and is pushing fibre hard. Even though fibre is being built out as fast as possible, in an ever increasing list of communities, it will still take time.
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Yes I have been told the speed will drop back for various reasons. My service has been stable and predictable, speed tests match the line speeds minus the expected overhead. Upstream line rates appear to be aggressively dropped and arguably cautiously increased. I hazard a guess there is direct correlation to the upstream SNR. Perhaps an upstream minimum SNR target has arbitrarily been established which would be now impossible to obtain on my lines. The rub is, with the exception of extremely hot summers days, I did have very stable 3000kbps on each line for well over a year with the same SNR currently on my lines. The major difference I observed was the modem upstream line rates were operating closer to the noise floor similar to the downstream SNR.
I should add I have kept with Telus for years because of reasonable Tier 2/3 tech support, very good onsite tech support and concierge customer service.
That said, Shaw Internet is still a very attractive option to me and their massive Shaw Go Wireless service makes me envious. It's just hard to pull the plug when one is familiar with the nuances of their current service provider.