I'm looking to build an m-itx PC, and the Lian Li case I purchased calls for a PSU with a length of only 140mm. This drastically reduces the available choices, especially for modular PSUs. Silverstone has a 750 watt version, but the reviews are lackluster.
 
Seasonic has a very highly rated 550 watt power supply that meets the length spec, but I'm curious if you think that PSU is powerful enough (single rail, 45Amps, 550 watts, Gold certified). The Extreme Power Supply calculator says it is fine, estimating a draw 0f 431 watts under 95% load for the PC I hope to build.  But I would like other opinions. Here would be the specs:
 
Gigabyte 787N Mobo (draws 65 watts, 127 under full load)
EVGA GTX 780 FTW (apparently draws 250 watts under full load?)
8 GB DDR3 1600 1.5V
1 samsung 256GB SSD- EVO 840
i5 3570 processor (no "k" version)
2 140mm fans
 
I have no plans to overclock the CPU, and there will be no optical drive or secondary hard drives.
 

I agree, you should most likely be fine, but you are getting close to be out of the efficiency band.  I don't know the specific output vs. efficiency for this psu, but being Gold certified, I would think the most efficient output band would be pretty wide.  Power supplies are most efficient in the middle of their output range.  At 431W calculated of 550W rated, that is about 78% capacity.  If I had to guess, the efficiency band would be about 40-70%.  Outside those ranges, the efficiency is reduced.  Lower quality units, the efficiency might really drop off, but in the case of that one, I would think it would only drop a few percent.

No, efficiency shouldn't affect the stability of the system.  As long as the components are receiving the amount of power they need, there shouldn't be any issues.  If certain rails will be starved for power, such as an overclocked CPU or GPU (which you aren't going to do), then stability could be a concern.  But yes, inefficiency would cause extra power consumption and therefore more heat.  At extreme cases of being way outside the efficiency area, the power supply could see reduced life expectancy.