How much should I care about the performance/benchmarks? I know that OnePlus 3 outperforms everything, including Samsung Galaxy S7, but on the other hand - will I use that power? I'm not gaming at all.

A quick look and it seems that spec-wise, OnePlus 3 is better than the Lenovo. Any reason to consider it?
We don't know anything about Lenovo + libhybris

Besides - I'm a touch typist So I don't think I need backlight

AFAIK most Android phones can handle USB keyboards without special treatment.