How does one calculate the profitability of GPU mining? As GPU mining made CPU mining non-profitable, at some point in time more specialized mining hardwares (FPGA and ASIC) would make GPU mining non-profitable. 

When is GPU mining considered not-profitable?

Considering $1000 rig, $0.2 per kWh and one year break-even period,
at which difficulty does GPU mining become non-profitable?

Or, what is the formula to find out "max non-profitable Mhash" for given parameters like, rig cost, power consumption, investment break-even period, difficulty, exchange rate, etc. (please ignore redundant parameters, and include more parameters if required.)

Or, whats the current "max non-profitable Mhash"?

Or, what is the required current energy efficiency (Mhash/J = millions hashes per joule) for mining to be profitable?

(energy efficiency; 1 joule of energy is 1 watt during 1 second: 1 J = 1 W*s)

For given parameters, Max. Non-profitable Hash Rate, is breakeven point for hash rate after which mining becomes profitable.
Using the Bitcoinx calculator, I found that a $1,000 rig with a 300 W power draw would need to have 750 MHash/s just to break even on power at $0.20/kWh at the current difficulty (3,438,909), exchange rate (13.13), and block reward (25). 

You'd never break even on the rig itself. Thus why GPU mining is on its way out.

For a more absurd example, I priced out a reasonable rig for ~$1,405 with three Radeon HD 7970s, the best mining card combo - 2050 MHash/s - on the market at the time of this writing according to the Mining Hardware Comparison chart. Its power draw would be around 850 W, according the MHC and a quick review of the parts. This rig would never be profitable at $0.20/kWh. Hell, at my electric rate of $0.07/kWh, it'd take nearly three years just for a return on investment. Prior to the recent halving, this same rig would make a return after just ~260 days.

More proof that GPU mining is on its way out.