Graphic Card Question

The price you paid for the single GPU is well worth it, but when you consider the marginal performance increase; the answer would be no it's not worth it.

To get the most of SLI you need to tweak the GPUs to get the most from them and you need the program you're running to cooperate. The latter of these two is probably going to be the biggest problem. It remains to be seen how well XIV will perform in SLI after the changes, but I can say confidently that it didn't really perform all that well prior to ARR.

In general, you're not going to double your performance with SLI so from that standpoint, it's not worth the money. There are some comparisons between SLI vs single GPU setups that will make a case for SLI being better price vs performance, but people often look at the wrong indicators when comparing benchmark scores. Your lowest FPS value should always be the deciding factor as it indicates performance when there is the most stress on the GPU. Consider that if you value benchmarks or using them for comparisons.