I think the addition of the D3s in about a month will be interesting. I have two thoughts:
1) I suspect the current rise in difficulty is due to Bitmain mining with their own D3s before they ship them out. You know, "testing" them. Recently the network hash rate has increased 15 Th/s AFTER the announcement of the D3. Bitmain mining with their own supply is the only scenario that could explain this. 15 Th/s is equivalent to 1000 D3s, or thousands upon thousands of its next closest competitor. There is no way that, after Bitmain made their D3 announcement, people ran out to buy Baikals or iBelinks in droves, knowing they would soon be obsolete. If anything, I would expect people to try and unload their old hardware before it becomes totally useless, thus difficulty should drop. For Bitmain, running 1000 D3s for one month would earn them roughly $5,000,000 USD. That's pretty tempting. I hope this is what happened, because it would mean that the D3s have already hit the network, and thus the meteoric rise in Difficulty that everyone is predicting on Sept. 15 has already happened, at least in part.
2) Once the D3 and A5s hit the network, mining Dash with anything else will be pointless, and presumably owners of older ASICs will mine a different X11 altcoin at that point. They will then leave the network, which will combat the Difficulty rise caused by the new heavyweight ASICs. I'd guess there is at least 15 Th/s worth of pre-D3 hardware on the network currently.
What do you think? Hopelessly optimistic?