It's true that SLi GTX 980Ti has more driver overhead than using a single GPU, but other than when using multiple cards, CPU load is relatively constant, regardless of in-game settings. What this means is, if an FX-6300 is capable of only providing 31fps average in a specific scenario with a GTX 980Ti, it will also only allow for 31fps average with a single GTX 750Ti or R7 260x, regardless of graphical settings. You could lower the game down to 800x600 with minimum graphical settings, and that CPU bottleneck still doesn't go away.
Yes, an FX-6300 provides an adequate experience in the vast majority of games, but an i3 is faster in most games, costs about the same, and has a better upgrade path, so if there's the option of an Intel CPU, a builder should take it.
That's all I'm trying to say.