We play with littler and faster when time allows. 17 caliber 15-18 grainiers busting 4,000 fps
The next rifle in the works is a 19 caliber either a 19 Badger or a 19-223.
Fast is fun...
The problem with pursuing pure speed with lighter bullets is that the BC drops fast and the speed you had disappears quickly.
So you need to consider the range at which you want "speed" because if the range is long enough, a slower heavier bullet is actually "faster" and "flatter."
If you want fast a flat in an AR, you should be running a .204 Ruger or any of the 20 cal variants (VarTarg, 20 Practical, etc).