Yes, a huge improvement, will post it in the evening.
EDIT: It was not a huge improvement. I decided to redo the tests with more rads to lower the water temperature (now it was around 35°C), to properly monitor the temperature of water incoming in the block (I have the water temp sensor that came with the pump and also have the Corsair controller and monitoring box).
The trouble is that I do not have any test computer and I did not want to mess too much with my only functioning PC (now air cooled in a small case).
Anyway, the result was that
the best performed the flat block with the plastic insert with the small slot. It seems that concentrating the water flow to the place where it matters most helped.
My one randomly chosen smaller slot size (1,3 x 26mm) means that there is most likely a different size which will perform even better. And such optimal slot will of course perform best when it will have a proper water guide leading to it.
While this block is considered to be one of the best performing blocks available today,
I believe there is still some room for improvent:
1) by making a flat version for people who have flat CPUs (easily doable by using thinner inserts)
2) by concentrating the water flow more to the place with the highest heat flow (easily doable by cutting different slot in the insert and redoing the water guide)
3) by possibly rearranging (optimising) the cuts in the base for this new water flow (the cuts are probably done with a CNC machine - just reprogram it)
I bet the engineers in Alphacool will be jumping up and down and shrieking with joy like apes in the ZOO when they get their bananas, after they find out, that they can improve their product even more...