Not sure you can enable/disable Nat Accelaration from the user interface. Is AI Protection and QOS disabled?
I just did my Speedtest again. Using IPv6 to the Rogers server I get 10 mbps upload. Using Telus 30. I wonder if there's some funky IPv6 routing issue going on? By the way, I'm still on .32 firmware, not .33 .
You know, that's a rather interesting observation. Maybe, for any situation where there's either slow speeds, inconsistent speeds, or micro-disconnects, a temporary test recommendation should be to run a ping test to the CMTS, using two test cases, one with IPV6 running and one with IPV4 only where IPV6 is disabled. Maybe that test should be run for every modem, just to see what turns up. You never know until you start looking at the data.
In my case, if I use any other speedtest.net server that supports IPv6 (such as Innsys, Teksavvy or Telus) I get my proper upload speed of 30 mbps. Only the Rogers server gives me 10 most of the time on IPv6 - but 30 on IPv4.
Are we discussing the white Hitron CODA-4582 exclusively, no other modem? If so, wonder if this has anything to do with the update to firmware version 7x, currently at 22.214.171.124? That uses an updated kernel which is supposed to support OFDMA (DOCSIS 3.1 upstream).
Have you tried google fibre's speedtest: http://speedtest.googlefiber.net/
I do have the white CODA 4582. I'm on 126.96.36.199 firmware and on the OFDMA upstream profile.