As a catsailor turned mothie, in 8-12 the moth is fully powered up and really few boats can match it in speed. On the times I sail against cats, they just can't hold the same pace. Have overtaken a Nacra 20 on a short downwind leg(about 0.6nm) when I even gave them a 200m headstart in 10 knots of breeze.

Once the breeze gets to 15 you really need to change a lot of gears to keep the boat in control. This involves adjusting ride-height on the fly between upwind and down. If this is non-adjustable, then set to fly to high upwind and you will be ventilating downwind which is slow and usually means lots of capsizing. Also the rigs become very overpowered and everything is on drum tight.

In 20+, it is another world. The boat just takes off as soon as you sheet in. You have no time to relax, no time to get yourself ready, it is just launches regardless of whether you want to go or not. When you have it under control it is amazingly quick. 25 knots downwind is easily achievable.