At 99% usage, it's the same as a GPU being used at 100%. Depending on what games that you are playing, Your 1070 does not need to use all of the available VRAM, it will only use what it needs. And for the GPU usage going down at 55-60%, it's likely the game that you are playing from the result of poor optimisation, this can cause other types of bugs/glitches such as frame drops/stutters. For the temperatures, those are the normal ranges that you would expect from a 1070, as higher end cards tend to use more power and therefore a higher heat output. If you are that worried about your GPU's temps, you can try to ramp its fan speed, or improve the airflow within your case.
There is absolutely no difference made in regards to FPS if you try to switch between DisplayPort and HDMI. FPS numbers are heavily dictated by the performance of your graphics card.
Why? why do you wish your GPU to reach 100% usage and 100% VRAM? I understand you wanna see your 1070's full potential, but at that point, the GTX 1070 will be running at full speed, increasing heat output, in which you were very concerned of. Well I guess that answers why your GPU gets "hot" doesn't it?
By the way, since you have an dual 1070 in SLI configuration, some games will not support SLI, so sometimes it will only use one GPU. With SLI, there are scaling issues, which can affect FPS.
And as for bottlenecks, no, there is absolutely no bottlenecking as far as I am aware with the specs provided. That processor you have there is still very, very powerful, and can still be matched against the Skylake i7's.
Overall, this is all normal! Nothing to be worried about!