i think it will be very difficult to ascertain, for example; my latest addition to my rig arrived last week, and since powering it up, i've now powered down other parts - AM, BFL and a few garden blades... sensible people switch off when it's no longer worthwhile, only the die hard will keep all their equipment running regardless of BTC price. A good indicator might be to watch ebay to see what people are selling off... I doubt we'll ever be able to ascertain true values on the network, one might assume lots of things, and manufacturers providing numbers is merely that, their own assumption that all of their customers are still running all of their ASICs. I would very much doubt that BitFury have anywhere near the amount of 40%. possibly still 30%, but i'm just pulling numbers from thin air, in that.
i think we might be able to group the systems in easy ways.. by their TCO - total cost of ownership... i.e., purchase price of the mining system, AND its power consumption, because those two factors ultimately determine their on-going profitability, and over time the latter (cost of operation) will become more important, as the network TH's increase yet the cost of power doesn't decrease.
the older systems (the bfl 65nm's, older avalons & asicminers etc) were all built using chips that consume multiple watts per GH (some as high as 6w/gh), are all being turned off or phased out (even as we speak) as they're no longer power efficient and the cost of electricity and price of bitcoin make them uneconomic to run. BUT the sum total of all of those 'multi watts per GH' systems ever sold, is still in the low single digit PHs (lets be generous, and say 5-10 PH's, and i think closer to 5 is likely)
thus even when people decide to switch off every single 'multi watt per GH' bitcoin miner, its will hardly make a dent in the network size because the growth today is already more, in one month, than the sum total of all 'multi watt per gh' systems that exist.
What you're left with (when all the older/inefficient miners are switched off) are all the newer systems, as sold by BitFury, KnCMiner, Cointerra, AntMiner, Innosilicon, etc... that are all currently operating around 1Watt/GH system power (give or take)... and thus are much more efficient than the previous generation, and also much lower cost to purchase thus there's many more of them and they're dominating the network... and once they've been purchased and deployed, few people will have an interest in turning those systems off until the price of bitcoin and the price of electricity make them uneconomic...
We'll wait til the next generation of lower powered miners comes out that may consume - perhaps <0.25 Watt/GH - that might emerge next year (or perhaps late this year)
i'm thinking it needs to be in the ballpark of 0.25 watt/gh rather than <0.5w/gh, because people need a big incentive to change behaviour so there may need to be a significant change in the cost of operation (power consumption) - or the cost of purchase - rather than a simple halving the power consumption like the newer systems coming this year might at best achieve).