Efficiency isn't really all that important, unless you are running your system at full load for a large portion of the year. Only then would you get back your money invested in a gold psu over a bronze. Efficiency is how good your PSU is at drawing power from the wall without wasting power as heat.
It is more important to get a PSU with quality components. The corsair VS lineup is an average PSU. It does have some cheaper capacitors in its construction. Your PSU does have enough watts/amps to run a gtx770. If you don't have the budget to get a new one, you should be ok, especially if you aren't overclocking. But if you are overclocking and if it's in your budget, I'd upgrade to one of the PSUs in at least tier 2B in this list...