Remember too that the iPhone 5S pretty quickly started to down-clock once a heavy work cycle came in.... so the benchmark scores before were almost a 'burst' but were likely unable to be sustained..
Apple are claiming (to be proven i guess) that the new 20nm process + other efficiencies are making the clock-speed more level
So yes... we have a 15-25% overall benchmark improvement but i reckon 'real world' will mean a better sustained performance with less drop-off
But also remember... every year ... 20% improvement is on the previous figure ...
So a 20% performance improvement from processor 'C' from processor 'B' is a higher gain than from processor 'A' to processor 'B' ... they're compounded each year..
We can only expect so much improvement year-on-year ... they made a big jump last year with 64-bit ... it'll be back to incremental now .... going down to 14nm maybe next year ....
I know it sucks to be us .... only having one of the most powerful (if not the most powerful) smartphones on the planet equivalent in graphics performance than a home console from not that long ago...
First world problems are so terrible.