So, I held of installing it on my 6S+ that's from the release in Sept' 2015. Battery is supposedly at 80% health.
I decided to have some 'fun' tonight and have just gone for the install via iTunes and an IPSW file. No restore, no DFU, just an update.
1st thoughts - wow, in a good way, it feels nippier! I use FB app a lot and that is definitely faster between pages, updates, clicking on a notification and that page loading is rapid, thats better than before.
Already knew my UK banking app wouldn't work, no surprise there, I've added that to the first post.
CPU reporting as 1848 Mhz (full speed) with battery at 43%. I will update this post as battery depletes and I keep checking the CPU speed.
Geekbench 4 CPU benchmark reports 2534 single core and 4412 multi core, this is as good as it gets for me. No drop off there

I like the 'new' (pmsl) fact that app store now shows the size of the waiting updates again. Why they took this off the front screen i'll never know. Useful info when you're on data from your plan and not on wifi.
Although I am using 2FA, messages in the cloud was
not turned on by default for me, I turned it on. Oops, that's cus I didn't open messages first and allow it to sync. lol