Interest payments down by over 50% in just the last 3 week 
That is correct. the average difficulty for September was 104,550,780 while this months average difficulty is going to be over 216,654,943 which is OVER a 100% increase.
Seems that money printing devices have seen somewhere around $100m in investment come in the form of 100% pre-orders and that investment has made the difficulty jump 30% more or less each diff period.
These are the risks along with manufacturer delay and BTC price.
And the BTC price has gone up with over 30% the last week, so It's not so big loss if we get 50% less BTC from Giga-/Teramining.
The BTC price will go up even more.