I can confirm this.
Yup, it looks like the actual speedup doesn't match the chains/day estimate. I never did a full comparison between hp9 and hp10 myself on mainnet because it's pretty expensive to do that. So big thanks to the guys who did. I did try to adjust the chains/day estimate to account for effects of extending the sieve. It's possible there are some bugs with that. Of course the estimate isn't fully accurate in the first place either.
With regards to the sieveextensions factor, will changing this value be reflected in the CPD? i.e. if I change it and I see CPD increase, can I take that as a positive thing, regardless if the increase % is inaccurate?
Do you have any other ideas for performance improvements in the pipeline?
I agree that it would be nice to have a stable metric. The issue is that if the metric is broken then it's simply misleading. The 'sieveextensions' parameter is reflected in chains/day but I have a feeling that it may be broken. And if it's broken, then you can't really trust it.
There are still things on my to-do list but I'm not sure if there's going to be anything big anymore.