Is there a way to go fastpath on downstream ?
I usually suggest people cap their sync a little lower, reducing errors. This usually makes DLM remove interleaving and return the line to fastpath.
You must have a pretty noisy line because you're already technically capped being on the 40Mb package with an attainable in the 70's.
You could still try capping the line for 8-10 days and see if that does the trick. If ES (and imo FEC) numbers are low enough DLM will return the line to fastpath.
The command to cap the line on the Zyxel is
adsl configure --maxDataRate 35000 10000 100000
That would cap the line at 35Mb down, 10Mb up. The last figure can be left as 100000 whatever cap you set.
If ES are high enough you may find the line just goes straight back to being interleaved a couple days after removing the cap. Some lines just can't hold fastpath with the DLM thresholds.
It would matter who the ISP is too. Different ISP's use different DLM profiles.