While experimenting with my TD-W8970 (VRX 268 chipset), I found that an older driver version just can't load a newer dsl_vr9_firmware .bin file. With the 8970, the dsl driver is a kernel module - dsl_drv_cpe_api.ko - and can be unloaded, so I tried loading a newer driver module (from the TP-Link 9980 firmware). But that didn't work, it seems you also need to update the Lantiq MEI driver.
I think the newer firmware images either just won't load, or may not work properly, unless you also update at least the lantiq dsl driver, mei driver, and control application. The trouble is the OpenReach GPL source code package is fairly old and doesn't seem to have ever been updated since it was first released.
What are the versions of the lantiq drivers in the latest OpenWrt?
Some newer lantiq sources have finally turned up on the Internet:
https://github.com/xdarklight