HST at 140 was 18mm HST at 165 was 17.5 mm so the tension at 165 is higher.
My mistake, I had it backwards. But that is very close to being the same amount of tension.
If we look at your latest efforts in reverse, going from 140 to 165bar only got you an increase of 40fps.
0.5mm isn't much of an increase in hammer spring tension and 40fps is barely over half the gain of what I expected you to get given the significant pressure difference.
I still have to wonder if you're getting sufficient hammer strike for 155bar or higher setting.