TL;DR - - - - > I got good batt life wt my S8+
After using my phone for a week, actually I think my s8+ has darn good batt life. I can achieved at least 5.5 to 6 hours of SOT daily before charging it when going to bed, wt at least 15 to 20% left. Coming from a note2 user (whom I be been using it for 4.5 years), whoa.. This phone is great!
Here's my settings / config
WiFi, Bluetooth: switched on whenever I have to
GPS, sync: most of the time. Only switch it off when batt is low
Screen resolution : the highest 1. WQHD+
Performance mode : optimized (rekomen dead)
Brightness : auto
Widgets : only Samsung standard weather widget. I don't like lotsa stuff on my screen.
Theme : I got a dark wallpaper and dark theme, both downloaded free from Samsung theme store. I love em hitam, just as my favourite colour

Sync apps : usual lor.. FB, email, etc. I allowed notifications from all my apps (I doesn't know if it's matter)
AOD : always on. Bloody love em cuz I don't have to click my phone in order to tell time and etc.
Activities : frequent Fifa mobile, fb, emails checking, msgs (whatsapp, LINE), reddit, chrome for surfing.. Etc
Might not be consider a heavy user, mb (?)
And learning from this forum, the 1st thing I got my phone was to do a hard reset.
As I m writing this, my SOT is 4.5 hours with 32% left
I am using a Spigen Neo Hybrid and waiting for my X-One SP (ordered online). I had dropped my phone back 1st to a floor from approx 2 feet that day cuz tukar seluar. Thx God the case was of cuz doing what it's supposed to do la.