My 1070 sc black edition has a supposed stock boost clock reading of 1784 but when i game, gpu-z states I'm running at 1999.5 max clock.

Can someone help me understand this?

I do have power target maxed in the precision xoc app. Is this affecting clock speeds?

Power target can increase the clockspeed - GPU Boost 3.0 will automatically increase clockspeed based on power, voltage, and temperature.
 
What you are seeing is normal, and a good clockspeed without any manual overclock!

Thanks. I thought that might be the case. I've actually added 150mhz without any voltage increase besides the power target.

This seems strange since I'd be at 8% increased voltage on my 1060 at this point.

Any idea how this stacks up to other 1070s? It seems good.

Not sure if I should go higher.

The low end for any 1070/1080 manual overclock is 1950MHz, the median is around 2050 MHz, and the top end is around 2164 MHz. 
 
There are some outliers that are above or below, yet that that guideline holds true for almost all 1070/1080 cards.