Their top-tier package in my area is 35/10 which is what I've opted for.

Presumably this will be upgraded to 70/10 or 70/20 when their network allows.

All Openreach fibre cabinets offer three products currently

40/2 - cheapest
40/10 - costs roughly 50p more per month wholesale
80/20 - adds another £1 roughly to wholesale cost

Which you are allowed to order is down to the rules created by each individual retailer, with most not even offering the 80/20 service if the estimate is below 40/10 for a line.

There is no easy flip a switch upgrades on the way to bring 80/20 to those can only get 40/10 or slower because of the distance from the cabinet. There are some upgrades in a year or two that may boost speeds by 10 to 20%, but the next major speed boost will be smaller cabinets being deployed closer to peoples homes and that is some years away apart from a limited size roll-out.

I'm receiving 80/20 from BT currently (well, 70/15).

As far as I could tell Talktalk didn't offer anything but 35/10 as their maximum for my line.

I may then be pleasantly surprised on the move and experience similar speeds? Or do you think they're just not offering 80/20 to me full stop?

I have bought their "Fibre Large" which does say "up to 76mb down" on this site: https://sales.talktalk.co.uk/product/fibre

The confirmation email says:

Fibre, Broadband & Calls
Your estimated broadband speed is : Download: 35Mb Upload: 10Mb

I've just done a speedtest on BT and am getting 60/15.

Any thoughts?

An update which may help other people in my shoes.

Talktalk got back to me and confirmed their estimate was incorrect, and I would receive equivalent speeds to BT.

The switchover happened on time and with no engineer input. I couldn't get the ECI modem and my router talking to TalkTalk at all at first so gave up and plugged in their bundled Huawei 635 router which worked out of the box.

Unfortunately sync speeds were about 20% lower than BT - I was connecting "real-world" at about 50mbps as opposed to 65mbps.

Trying to configure the ECI modem and router again, I eventually reset the (TP-Link WDR 4300 running OpenWRT) router to factory settings, and on reboot the router connected automatically using DHCP and aquired an external IP address.

It turns out (?) that Talktalk don't use PPPoE, and DHCPing through the modem just...works. I'm assigned a public WAN IP for the router, rather than an internal one, and I haven't stumbled over any double-NATting issues thus far.

Did the ECI modem restore the higher sync?

Ah yes, the sync speed is now equivalent to the previous BT speeds.

I have read from various sources that the ECI modems were supplied specifically for DSLAMs with ECI kit, as Huawei modems have sync issues.

It's curious, if this is the case, that Talktalk are now only providing the Huawei combined router/modem box.

The HG635 actually wasn't particularly awful, but there is no excuse for 10/100mbps ports on LAN these days.

In addition the firmware was locked down, and although there were some useful features it seemed under-resourced for the sort of network traffic our house experiences (at least 5 connected devices, HD streaming over wifi, and so on). Overall network performancy was significantly improved with the 2-box approach.