Before my arm got too achy i threw flat shots FH with a 166 Beast to 300' tops. I can't throw hard with my injured arm. The speed rating is a rough estimate that deals with the drag of the disc slowing down it. There is another more complex way of looking at the speed rating which incorporates the turn and fade characteristics. Speed can also be seen to be a power requirement where the disc stays at the same height to undetectable dropping. If it flies flat and that depends on the fade and turn numbers too. The faster the disc the more power you need to put into it to make it fly flat to not drop early and fast to be possibly shorter than a similarly fading slower disc thrown higher.
Another view is to look at the apex height of the throw. A faster similarly gliding disc at the same height goes faster if it doesn't fade out. So when you need to throw far and under branches you will need to step up in speed but to a disc that won't fade more than a slower disc.
Flat shots need running on the center line of the tee and planting each step on the center line. Anhyzer needs running from rear right to front left with the plant step hitting the ground to the left of the line you're running on. Hyzer is the mirror of that.