You can like what you want, and recommend what you want but where is this imaginary 2% difference coming from?
At 50% load the difference is 7%.
https://en.wikipedia.org/wiki/80_Plus
And you btw, you did not recommend a 550w PSU, I did. You recommended a 520w,
Based on the coolermaster site, the Seasonic 520w would be running at around 73% (moderate load, more at full-bore) and the Rosewill 550w would be at 69% at the same load accentuating the difference. At only 73% load the seasonic will be around 83.5% efficient. At 69% the rosewill would be at around 91% efficient.
FOR A DIFFERENCE OF 7.5% NOT YOUR 2%
So to provide 367w the Seasonic would have to consume around 439.5w
Math is simple 367 / 0.835 =439.5
Subtract the power delivered and you waste 72.5watts as heat that your PSU it putting in your case.
With the Rosewill to provide 367w it would have to consume 403w which results in 36w of heat.
The difference is also simple math 72.5 - 36 = 36.5w. Or just over 6 times as much as you stated.
So assuming the rest of your math was right, then the pay back would be in 7.5 months. But I don't buy (or recommend) efficient PSUs to save money (they do save you money) but because heat is "bad" for computers., Having the equivalent of a 40watt light bulb in your case dumping heat is not good. If you are worried about longevity of your PSU (and/or just wish to avoid surprise repairs) a surge suppressor with a line filter is a great investment.