Yeah, it will surely be unimportant in march next year when every watt will be counted if your miner is plugged from the wall or not 
Perhaps, by that point you could always underclock kit, or move on to next gen - nothing lasts forever, especially at the rate at which Bitcoin mining tech is racing towards the smallest feasible die shrink, if the BTC price justifies a worthwhile investment Regarding your idea about undercloking: It is not an option. Electricity costs eat up the generated BTC profits so fast that you have no window to mine with underclocked device. Look at KnC profit calcuclation for example:
http://mining.thegenesisblock.com/a/c14d336591
See how electricity costs kick in march next year and there is no profitability only month later? It's not only because off diff increase but also the watts you are spending. Reducing the hashrate for that month won't give you essentially nothing. That's why KnC energy 100% inefficiency compared to other 28nm manufacutrers and 60% compared to Bitfury will be very important in a few months, regardless it is not an issue this month. You can always claim that both Hashfast & CoinTerra lie they are 100% more efficient than KnC, but that's also be claimed for Bitfury and came out to be false, they are really that efficient.