Overclocking EVGA GTX 970 ssc

I'm working on squeezing this thing for every frame I can get.

I started out on GPU tweak which was acting weird for me, because it lets you adjust the "boost clock" but when I'd apply a setting, my card would boost way past that number and crash the driver, for example if I set 1480 boost, the graph would show something like 1600+mhz at the time of the driver crash. This program was obviously not meant for my EVGA card, though I feel like I prefer it's layout.

So I moved onto the correct tool Precision X 16, now things were working as expected. Factory is 1190mhz which boosts to 1380mhz in the graph. I got only as far as +80mhz (boosts to something like 1460mhz in Unigine Heavan 4.0 benchmark Utility without artifacts. So I started adding voltage just to experiment, and 50mv later it still hadn't overcome the artifacts at +85mhz, the card simply does not respond to voltage at all. It's not hitting the power limit or temperature ceiling, and the graph shows the clock is not throttling.

Is this unusual? am I overlooking something? Or is it just the way it goes.

That program works fine with EVGA cards. The behavior you are seeing is normal and will happen with any program, even the EVGA one as that is how boost works. The value you are seeing/setting it to is the MINIMUM boost. If temperatures and power use are in order it boosts past that value.

Ok.

So if +50mv doesn't get me another 5mhz on the core, I'm basically done right?

If you are certain it is not hitting the temp/power limit as you mention, yes.