FWIW if anyone else comes back to this I spent a little time last night and I've addressed my font rendering issues by;

* Removing an errant font-config lcdfilter configuration entry (this was a left over from when I was running "infinality" and should have been removed)

* Switching font from Noto Sans to Ubuntu

Its not perfect, but its a lot better. I'll probably play with oversampling (rendering X at a higher resolution and scaling down like OSX) at some point to see if I get the quality I want.