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.
It's a big loss of bitcoins, for instance, I invested a lot of coins in gigamining shares & ended up with substantially less when I claimed for the 1530 shares that I had back in Feb, I got just 245 btc in back interest instead of the thousands that I'd invested, then I paid 382 btc for the paid upgrade to teramining on giga's assurance that this time it would be different, so far it's paid 173 btc less the 70.36 btc I would have received by sticking to the free upgrade, so 103 btc gained for a 382 btc cost & interest has slowed to 3.60 btc per week falling by 50% each 3 weeks on the latest figures, I don't see how this can ever cover the cost even with giga's assurance that he somehow intends to make it right.
The only honorable thing that I see now would be if he admitted the paid upgrade was a failure for those who took it & refund the cost of it, less interest paid out, but plus the free upgrade interest, which in my case would be 382 - 173 + 70 = 279 BTC or to explain when & how he intends to make this investment right for those who have supported him all along.
The 245 btc that I got back in Feb plus the free upgrade coins of 70 btc = 315 btc, these would be some of the very few coins that I had in my cold storage after the scams of last year, like bitcoinica, GLBSE etc, instead of that I am now 279 btc down atm just from taking the paid upgrade & earning a rapidly declining interest that can never hope to recoup this.