Lighter is better for almost everything (as you said, mainly acceleration/deceleration/turning), but in my limited experience lighter boards bounce more in chop (all other things being equal).
I do not race kiteboards
What i think about is that for strap surfing at Jaws ...
...Riders are going at high speed down the bouncy face
The surfers prefer to make their boards heavier on purpose to track better and minimize bounce
Maybe for racing they should assign "handicaps" for "board+rider weight+ kite size"
A calculation where you would multiply the racer time from start to finish X the handicap number
In my racing sailboats (dinghy) years
All different type sailboats would enter the same race and using their time X handicap
You would determine the winner
So you could have in that race a 505,470,Tornado,Flying junior, Fireball and a Flying Dutchman
They all had a chance to win the race
From the expected fastest (Tornado) to the expected slowest (Flying junior)
Maybe one day it will be how kiteboard racing will be