HI There, the biggest change in the smaller boards is the outline/the way the boards can stay more upwind/90° vs the previous generations - so the older boards were very good at GPS/Speed sailing, but a bit harder to sail on a tighter course, like for example Defi Wind. The newer straighter outlines allow an easier "tight reach" to be sailed and still have excellent top end speed.
The changes also allow you to use a touch bigger sails so you can apply more pressure on the board with the slightly wider tails/straighter outline? The 80 works best with a 6.3 as the biggest size for normal slalom (GPS can alway be much bigger if running downwind), ideal sizes 5.0-6.0. The 90 biggest size for normal slalom is a 6.8/7.0, ideal sizes 5.7/6.3. This is always dependant on sailor level and the type of sail/local conditions somewhat.
So far feedback is excellent on ease of use, gybing, control and as mentioned, ability to hold a tighter line/course.