One of my favorite boards is the Hydroshort. I've had it in every construction possible, different lengths/dims, and both tail variations. So I'm pretty confident on how the board rides. I bought one of Tomo's personal boards that had NO QISC - just a single running through out the tail. On the 1st session I could immediately tell the difference.
The one without had almost no lift. It felt like the board was sort of stuck to the water... and the board did not have the same type of rail control. On bottom turns and top turns it almost felt like the rail didnt have the same grip - like it wouldn't immediately engage with the water. Like there was a slide or a slight delay. Basically felt slower and didnt turn the same.
All of these feelings were somewhat subtle and not necessarily bad, but I did notice the difference. Which means that the concaves though the middle and on the outside rails do provide a benefit to the design (at least the Hydroshort model). I message Daniel about what I was feeling and he said he had the exact same experience.
I still ride the HS without the QISC and love it for certain conditions, but it was insightful to experiment and feel the difference.