I have now tried the board again today.
This time it was in messy waves, 3-4 foot surf and wind changing up and down between 7-11ms. I rigged my medium kite a 12m Rhino7 in order to cope with the high wind periods. I was (at least with my normal board) severe underpowered in the low periods (remember 110kg!).
In these conditions the underground really shined: I had the cability to cope with the bad wind and it was still easy to cope with the board full powered. When full powered the board functioned best when I edged it harder. It did not like full blast laying flat.
Then it was a lightwind board required a tribut though: A little extra care at applying backfoot pressure was needed, but I did this more and more intuitive as the day progressed.
Compared with ny previous lightwind boards spleene session 2006 and a Nobile 666Ext 143x48. This board was not as tireing at higher wind, probaly because it is a bit smaller (at least felled so). So for me it is a good compromise. After my first session with the board I was not convinced that it was a good board, but now I am. I hope it will grow some more on me.