just seeking confirmation; once you recieive this armor it will stay that tier (lvl range) forever and it's impossible to get the final tier?

I don't understand what you're talking about. Take a look there: http://content1.uesp.net/wiki/Skyrim:Nightingale_Armor

In my experience, yes. I've even tried duplicating it to see if I could reach the upgraded variation, but to no avail.

ok thanks mr darcy, I wish I'd have checked the wiki before accepting the armor /slit

It can be upgraded, I was able to with no problem. I can't upgrade the Nightingale Blade or Bow though, my level was too high when I got them.

it can? lvl wise? not with smithing and void salts that is

I am sorry, I misunderstood the question. No, it will stay the tier it is when you receive it. I though you meant tempering it, my apologies.

yea sorry, I should have been more clear, thanks anyway