I spoke to soon - my downstream IP profile has just changed to 41.83.
I swapped back to my HH3 (hoping I get the firmware fix for NAT loopback sooner rather than later and not using the gigabit port this time). The reconnection of the PPP session must have seen me benefit from a better sync rate that probably occured overnight. For the benefit of others, I did not (and will not be) rebooting the modem as that is clearly resyncing speed as it should be.
I'll report back after my 10th day where I end up.
So to recap, before I went for the infinity up to 80/20 service upgrade: I had a 38.71 downstream IP profile. I was acheiving 38.5mb/s downloads via the Namesco speed tester with a around 6.3 upload. My very original Infinity service (going back say 14 months or so) was achiving ~25mb download and 7mb upload, but this changed I think when the faster infinity service was introduced in general (although I had not signed up for anything different).
Until very recently, the Infinity prediction had not suggested any increase at all for me. However the Infinity site recently started showing an estimated 47MB down and 8 up. Compare to my 37.5 down and 6.3ish up, I went for it.
My 10 day stabilisation period ended Friday just gone. For a good few days into my stabilisation period nothing changed, my IP profile had not moved from 38.71 (although the bt speed tester site reflected my upgrade due to the max upload changing from 10 to 20). After a reboot of my hh3 (not modem) about 3 days in, the IP profile changed to 41.83. I left all alone after that, but not long after (maybe next day) the IP profile was back down to 39.5 where it stayed until the stabilisation period was up.
Yesterday (day after my 10 days were up), I restarted my HH3. No change in IP profile. I decided to do a reboot of the modem too before I left it all alone. After the modem restart I am now on a downstream IP profile of 41.73 (am am still today).
So my upgrade has seen me go from 38.5 downloads to over 40. However my upload has dropped (seems to happen with ever downstream increase I see) from ~6.3 to 5.8.