My S7 is the Exynos version. I use Battery Widget graph to monitor temps. Max temp with high CPU use is never higher than 34c and goes down to overnight idle of 19c ambient.
If I charge immediately after heavy use 30-32c , temp drops while normal cable charging to around 20c. The opposite happens while wireless fast charging with temp rising during charge to around 32c.
I do believe the Exynos version is more battery efficient and runs cooler than the SD version. I will only get the S8 if I can get an Exynos version.