Sorry I was not on e-mail yesterday.
Indeed Timo did a good job explaining -- when you are
1) Inside ntoskrnl
2) In guranteed KernelMode context
It is better to use Nt for performance, as Zw provides no advantages. Especially in the boot path, eliminating system calls is a good thing. That's why we should prefer to use Nt whenever possible in system threads or boot code. In fact not even Windows always respects this "rule"/benefit, and we could do better as well.
As for coding style, indeed, double parantheses are something I'd like to see. I also personally hate one-line if statements without braces, however I understand that the coding style does not enforce that... HOWEVER, if you see a file written 99% by one dev with 99% one style, it seems a bit offputting to add a few extra lines of code that use a different style. The point of coding guidelines is consistency.