I just can not see how we can be at a diff of 317 and hash of 2.8 x 830ph = 2324ph
I don't think of it in terms of hashrate being a constraint, I think in terms of wattage... in my observation, availability of hardware doesn't seem to me to be the slowdown in the rate of growth; capacity for power does, since the megafarms are the ones driving total network speed now, and power/cooling are their primary concern when it comes to planning growth.
Worst case: Assume half the network is .5 watts per gigahash/sec or worse, the remaining is .25w, and that all miners with .5w+ devices will be be turned off as unprofitable and replaced with .05w devices in the next few months. For the same power capacity, half the network goes up 10x, which means the whole network goes up 5x to about 4,200 petahash/sec. (Actually, it's closer to 5.5x, but I'm generalizing here anyway). That's the maximum power capacity of the current network in the short term. The restriction then would be the price of the sub-20nm devices we're being promised...if their prices are 5x higher than the .25w devices, there's no incentive for the .25w miners to adopt early by dropping their .25w devices for the .05w stuff. If their prices are less than 5x, then Katie bar the door, because there's going to be a race to the halving - and past it - like the widespread introduction of the first ASICs. If the exchange rate of BTC keeps pace with (or does better than) the rise in difficulty *after* the halving compared to the price they paid for the equipment before the halving, then the halving will be transparent to their bottom lines and it won't be a factor for large farm adoption of .05w devices. If the exchange rate doesn't keep pace at the halving, then network growth should effectively stop until the older devices are paid off. (Keep in mind, though, that it only matters to current revenue generation - miners that have a large stash of mined Bitcoins held in reserve will benefit much more to the rise in price, giving them even more power to grow immediately by adding .05w devices to their existing .25w farms.) It's definitely possible, then, to surpass your expectations - in theory.
All that said, that is the worst case I see - I don't think there is actually enough manufacturing capacity with the existing vendors to deliver that much .05w hash in such a short timeframe. I personally think the non-manufacturer megafarms are waiting until after the halving to determine what risks to take next. I know if I had even 5% of the network, I'd be sitting on my hands right now hoarding BTC hoping for an exchange rate increase even more than it has in the last quarter... unless I was making my own hardware and already knew my cost for upgrading, then I'd be balls to the wall building more and more capacity in my farms. Poor us small potatoes, though...totally at the mercy of the hardware vendors deciding how much to mine for themselves and how much to offer for sale (and for what price).
Lots of moving parts. Fascinating, and in some ways scary.