Heat is the mortal enemy of all electronic components. The cooler they run the longer they last. My solution to the dilemma you have articulated is to use an over sized PSU which will never run over 50% of rated wattage. This is an insurance policy that most builders ignore. BTW, I'd stick with the Seasonic but I recommend buying the highest wattage you can afford. FWIW, I've used a top mounted PSU for all my builds without heat problems and I live in central Arizona.
Not really. Most computers have the PSU right there at the top because it's the way OEM computers are built. Although it's true that PSU's are most efficient the cooler they are, the air it will suck from the case won't be too hot, 40ºC maximum. Next time you can buy a case with a low mounted PSU, but I wouldn't buy one now for the single purpose of keeping the PSU cooler. Heat doesn't age a PSU, load does. The more you load the PSU the fastest it ages. Depending on that load, you can expect a 10-20% drop in maximum power each 1-2 years.
Conventional electrolytic capacitors degrade over time due to heat. Poor quality electrolytic capacitors degrade quicker and tend to be used in cheap PSUs with shorter than industry average warranty periods because they keep the cost of the PSU down.
The amount of heat that a PSU emits is dependent on its AC-to-DC conversion efficiency. The 80 PLUS certification ranks PSUs based on their conversion efficiency but it is certified at a non-realistic ambient operating temperature of 23°C. Some PSUs power capacity will drop as the ambient temperature rises (i.e. derating). Reputable PSU reviewers will actually subject the PSU to a hot box test to determine any changes to the PSU's electrical characteristics due to the rise in ambient temperature. Get a PSU that can output its full power rating at an ambient temperature of 45°C to 50°C.
For a PSU that has the same power capacity rating and under the same load an 80 PLUS Platinum efficiency rated PSU will feel and operate cooler than one rated with an 80 PLUS Bronze efficiency rating.
1) Ideally you want the base the size of the PSU on:
.. ALWAYS pick a "quality" PSU, NO lower than a teir 3 PSU!!
... At Idle power is at least be approx 20% of rated value. For example if your idle power is say 150 watts thne max size should be approx size should be 750 Watts.
.. At max power (ie runing your most demanding game) then PSU load should be approx 50(ideally) ->70% (Max) PSU ratings. So if runing your "maxed out" game you draw 350W then PSU shoud be between 700 Watts and no LESS than 500 watts.
.. My choice would be a 650 Watt PSU. Idle - 150/650 = 23% and at load - 350/650 = 54% NOTE: In this case I would NOT go with higher than 650W UNLESS you are planning on going with a higher end GPU that ups your max power.
.. NOTE. PSU must be at least a teir 3 or above PSU. My 150W/350 W are based on an OCed i5-2500K and a 7850/7870 GPU.
" Heat doesn't age a PSU, load does" Heat is the Biggest factor on how long a PSU will last. The load, coupled with other factors will determine the "Heat". The other factors deal with the quality of the PSU:
.. Effeciency. This is the ratio of output vs the input power to deliever that output. The difference is what is converted to heat (Losses). The lower the Eff, the MORE heat generated at a given Load.
.. PSU cooling, is airflow and heat sinks used. Quality PSUs will use a better Heat sinks for components Plus a beter airflow across these components.
Bottom Line: Following the above, I have never had a PSU fail, even after 5 -> 6 years of use.