Do people notice slowdowns yes.
I think your comment is a bit harsh, I would rephrase that people may notice things slow down but have little idea or even the time to investigate as to why, so just get on with things instead of worrying about it, thats probably closer to the truth than not noticing.
The primary new technology openreach is working on is g.fast which is still pretty much the same as VDSL except over a bigger frequency range, so any improvements to FTTC DLM would probably apply to that product as well. Not to mention most of their customer base is on FTTC or ADSL and probably will be for at least another 5 years (with the rate openreach are progressing, and the products LLU's are pushing), so anything that cuts down on engineer callouts etc. is probably worth pursuing for them.
I know several people personally who are definitely not nerds, and have in casual conversation mentioned (to me, not other way round) about internet problems they had, whether its laggy browsing, dodgy streaming or just slow downloads. People do notice these things. My 71 year old dad as an example knows what an ip address is which surprised me, people do learn things.
This is obviously not going to be labeled as a DLM reset, it will be dumbed down in tech support language to something like "yes sir I think I know what the problem is, we will make changes to your line and you should notice an improvement in X days".