Hi DPI good or not good?
For me it is not fun to look at. It appears to be larger at “+25”, but not better. I went back to NOT use the HiDPI anymore. I took two screenshots of the same project including tracks, left, right and lower areas.
Lets zoom into it at 600%
The waveforms (100 vs. 125):
Track Controls (100 vs. 125):
Top Icon Bar (100 vs. 125):
We see that the Fonts render quite well at higher DPI (they exist in a vector format)
But the most graphical elements like the waveforms and for example the mute and solo buttons are sharp at 100% and get blurry at 125%.
To me this is an upscaling effect where the original image for the button is made for 100% and is scaled up for 125 with smoothing. I can reproduce this in paint.net, I am taking the first image from the Track Control (at 100%) and apply an bilinear upscaling to 125% and I get this:
I know, it is a huge task to change a GUI from pixel-based icons to vector-based icons. 
Thanks for reading
Martin 