The riser is a bad idea for this case. He won't get SLI unless the riser's are used on both GPU's side by side.
I also looked at this MB's manual.
View attachment 182582
View attachment 182583
Since you have a 28 lane CPU, you can only do x16/x8 at the most.
Back on topic, have you tried switching the top GPU with the bottom one to give it the best airflow possible as it just might be a toasty GPU and have you checked the GPU's ASIC quality? Try a few more things before you dip into your pocket. Could just be bad case airflow. I will say I'm surprised to see these type of temps on a GTX 1070.