ok i probably will get flamed for this
i tested the z5, z5c and z3+
the z5 has this sharper edge because its border is slightly thicker than the body itself. the z1 is the opposite. z5c has a nicer edge although the thickness makes it look stubby
the z5's camera is slightly slower than my z1 and even at iso400 8mp, it is slightly noisy (took pics of the objects beside the booth)
the z3+ fared slightly better, the AF and shutter is slightly faster and less grainy at the same settings.
I did a side by side shutter speed comparison between the Z3+, Z3, Z2, Z3C and surprisingly the Z3+ was the slowest, Z3/Z2 about the same and Z3C the fastest. I compared them 2 at a time of course.
The Z3+ was so slow that I can hit its shutter first and it still slower to take a shot compared to the others. The shutter lag was so bad that it put me off buying it even though I liked it so much

I have tried the Z5 but haven't done a side by side comparison but it is fast. Definitely much faster than the Z3+ which strangely had the slowest shutter speed among all the Z phones I have ever tried.
Your test showed that Z3+ had the fastest shutter speed whereas in my test it was the slowest. Strange!
Which demo sets did you try?
I tried at 313 and Singtel shop at ION. All the Z3+ had the same slow shutter