Using the Xperia Z1 Compact and Samsung S3 as reference points, I would rate one-hand usability of the Z1 Compact as 10, the S3 as an 8 and the Z2 as a 6 or 7.
However, there are ways to make it easier to use the Z2 with one hand. If you have Nova Launcher installed for instance, you can swipe downwards anywhere on the homescreen to pull down the notification bar; no need to reach all the way to the top. And if you install the Touchpal keyboard app, the app allows you to shrink the keyboard to one side of the screen for enhanced one-handed usage.
I'll be upgrading from a Xperia Ray, which is only 3.3"
But yes, I tried Z1 in a store and I found it very easily usable with one hand(seems like I have big hands in such case
) , the only issue I had was reaching the notification bar as you mentioned. Its not like I had to use second hand to reach it but I had to do hand aerobics to reach up there, so I was still able to pull it down using only my right hand.
Z2 is almost the same size as Z1.
And yup its true about the Nova launcher, I use it even on my Ray, the gestures feature is just awesome!
Talking of touchpal, I'm sure Z2 keyboard has one hand mode too, because I tried Z1 and it had the "one hand mode" where it shrinks the keyboard to a side as you mentioned.