2&4) From what I can understand, you feel that GPU's are no longer the "every miner's" tool. Unless I can cut operating costs or investment somehow, yet you aren't quite sure how FPGA/ASIC systems will fare in comparison. What would you personally consider a high power cost?
Right. Miners effectively now have the option of efficiency through MH/$ efficiency (the GPU choice) or MH/KWh (the FPGA/ASIC choice). GPUs are relatively cheap up-front, but are risky for those paying a "reasonable amount" for electricity. As adoption of power-efficient miners increases as a % of total miners, there may be less resistance against a higher difficulty and lower price. That is, where old data show miners seem to drop out when Bitcoins lose value (whether this is because interest in BTC drops or because mining becomes unprofitable is up for debate) - what we may see in the future is that when price drops without a difficulty adjustment, GPU miners stop mining, but FPGA/ASIC miners keep going because, in any short-term scenario currently conceivable, there's no reason not to.
If difficulty remains where it's at now, and the price of BTC drops to $1, there isn't much reason for FPGA/ASIC miners to stop mining, because they don't pay much to create the coins. Many GPU miners, however, would not be willing to pay $5 in electricity to produce a good worth $1. Instead, in the future, difficulty may stop decreasing, and only increase at a slower rate when price drops. Just as CPU miners are now laughable, GPU miners may soon suffer the same fate, and in the future, were the market to adjust to having the majority of miners using high-efficiency FPGA or ASIC rigs, those too may eventually be superseded by something brand new and even more efficient.
As for "high power cost" (which was a bad phrase to use, but I'll run with it) - I'd consider that any amount where mining on mid-range ATI cards is unprofitable, which I'd guesstimate is something like $.15/KWh. Using that definition, and assuming FPGA/ASIC cards continue increasing in usage as miners, "high power cost" will lower with time, perhaps in a couple years settling at something like $.04/KWh.
(Quick disclaimer: I wouldn't be surprised to learn I'm overstating the impact of higher-power-efficiency hardware. It's a pretty big investment to buy something which has little existing demand outside of tiny markets, and which costs multiple times more than GPUs. I have no idea what % of miners run FPGA rigs. Maybe 5% currently, maybe .05%. In two years, FPGA miners may only make up maybe 3% of the network's total hashing power. *shrug*)