If I'm understanding this right - you are basically saying the battery life of the 6s+ is over 2x better than the 6s. What are you doing between 9am-3pm that puts you between 20-30%? Considering that's only 6 hours, it seems like you'd have to be on the phone about 4-5 hours using LTE, right? Over the course of a work day, the average person spends what - maybe 1-2 hour on their phone?
Honestly, yes. That's actually pretty much exactly what I'm seeing from my 6s+ when compared to my iPhone 6 (to which I complained about battery life not being all that great when I got it last year on release day). On my 6, if I'm remembering correctly, I would get about 6.5-7 hours of usage with 8-9 hours standby, so I'd say that's pretty heavy use (this was primarily on wifi, actually). On my 6s+ I'm easily hitting 12-13 hours of usage with 16-20 hours of standby, however I'd like to mention that my usage is a little different now that I'm back at school. I'll explain more about this below. But yeah, it's honestly insane. On lighter days where I don't use my phone as much I've even gotten 9 hours of usage (easily could've gotten 10, maybe 11 hours) with 30+ hours of standby and I STILL had 14% left. I could NEVER get anywhere near that with my 6. Also this is without using low power mode at all.
My 9am-3pm time that I was talking about was the shift I worked over the summer. It was an extremely easy job that required me to sit at a desk for the entire shift doing very light work so I used my iPhone 6 nearly the whole time (on wifi) and would go workout on my lunch break which involved me listening to a lot of music for about an hour/hour and a half. By the time I got home at like 3pm I'd be down to 20-30% which would last me until maybe 4-5pm when it would either be nearly dead or at an uncomfortably low percentage (like 5% or so). Things didn't change much when I came back to school, but were slightly better.
I'd wake up for class around 9am, unplug at 9:15-9:30 and get back to my dorm around 3-5pm with ~25% battery. Usage times would hover around ~5.5-6.5 hours with standby being at around 8 hours usually before I plugged in around 6pm. On heavy use days (like the day of the iPhone 6s keynote) my phone was at 15% by 2:30pm. So yeah, not the best battery life. Also, these usage times were with using low power mode a few times, which did help a little bit. In comparison, my 6s+, with the same type of use at school as on my 6 along with same exact settings, apps, etc. (I restored from a backup of my 6) is never below 70% by the time I get back to my dorm. By the time I go to bed (which lately has been 2-3am) it's at like 40% (I use it very heavily once I'm done with class).
So yep, in my case I'm seeing double the battery life of my 6, often times even more than double. Hopefully this clears things up a bit. Sorry for the delayed response/explanation, I've been very busy with school the last few days.