Hard limit price is growing more or less when amount of HBD grows faster than amount of HIVE

It isn't the amount that is important, it is the value. The relationship is dynamic due to the price feed.

No no, price feed does not influence hard cap price, just the relation between HIVE and HBD amounts. Let's see how much exactly we need new HIVE to offset one new HBD for 10% hard cap price to stay the same.

Using h = HBD supply (corrected for treasury balance), v = HIVE supply we want to calculate x so new hard cap price with 1 new HBD and x new HIVE (left) equals old price (right):
9(h+1) / (v+x) = 9h / v
9(h+1)v = 9h(v+x)
hv + v = hv + hx
v = hx
x = v/h
Let's see how it is for 30% hard cap.
7(h+1) / 3(v+x) = 7h / 3v
21(h+1)v = 21h(v+x)
hv + v = hv + hx
v = hx
x = v/h
The same. But that's actually good argument for raising hard limit - when ratio of HBD to HIVE gets bigger, the x, which is its inverse, gets smaller, that is, we need to produce less new HIVE for each new HBD for hard limit price to stay stable.