I have a different take on it's value as a flat-water board.
I used the 9'10 Allwave as my "Go To" flat-water board for 1.5 years. I was 120 kg when I bought it, primarily for small waves and flat-water, because I already had a 9'6 Allwave for surf (now an 8'10)
I don't time myself and I'm not in a hurry. I just like to spend a few hours a day paddling if there is no surf.
I have paddled many times with friends on 12'6 race shapes and did not hold them back. We were not racing, just paddling along.... the 9'10 is fine.
Heck... now I paddle my 8'10 those same distances. Follow this link to see more about the 8'10 as a flat-water board.
www.seabreeze.com.au/forums/Stand-Up-Paddle/SUP/Naish-Mana-810-vs-Fanatic-Allwave-810/One VERY overlooked feature of paddling a shorter surf SUP on flat-water..... they fit into small creeks and tight places.... great for exploring the backwaters. VERY stable for picture taking, rougher weather.... Plus super easy to throw in the car..
I am a HUGE fan of smaller boards for casual paddling.
They surf great. Plenty of info here and the standupzone on that.