I ran the firmware listed in the link on my 877 for a day and I'm afraid to say it doesn't solve the ADSL2+ problem for me. Initially the connection is good and it remains good for several hours then the ATM interface starts recording tons of CRC errors, browsing becomes like treacle and we start seeing packet loss.
The SNR doesn't drop (as previously seen in other versions of the firmware) however the connection is unusable after several hours.
I have now gone back to using a Netgear DG834 for the line sync and getting the Cisco to do PPPoE.
Current ISP: Xilo LLU (via Cable & Wireless)
Previous ISP: Zen (for 9 years).