Why, exactly? You should always follow up a statement like that with the results you're looking for.
A faster burning powder wouldn't give you higher velocities than a slower one. It might give you less flash -- but that's way more affected by your choice of an effective flash-hider/muzzle brake than powder on an AR. What else does a quicker burning powder do for you?
Sam, a faster burning powder will help because you can actually get a full burn in a 10.5" barrel whereas something like CFE223, Varget will not burn completely, and you are just blowing unburnt powder out the end of the barrel, thus you are not getting the full velocity potential for your setup. One other powder that might work for you is IMR 8208 XBR. Short cut extruded, has about the same burn rate as H4895, but much easier to work with it, and you can get more of it in the case without heavy compression. With the heavier bullets youll need a little bit slower powder to drive the longer heavier bullets to higher velocity. A fast powder like H322 or 4198 might see early pressure signs with a longer bullet like a 69gr, but will work great with a 55gr.
You can control flash with the right muzzle device, but if you are getting alot of flash, thats a sign that you also have alot of unburnt powder flying out the end of the barrel.
This is one of the reasons I dont recommend long burning powders like Varget in 223 without a minimum of a 18" barrel and a rifle length gas system. It artificially increases port pressure on the gas system which over works the action, and the only real solution for that is to use an adjustable gas block. Hornady has an excellent graphic on using their Superformance powders in gas guns that shows exactly what happens but I cant find it.
With short barrels and heavy projectiles is a very fine balancing act. Longer barrels give alot more flexibility. My advice is this: Buy a pound of H322, H335, 8208 XBR and test velocity, accuracy, and flash. I know you will see the highest velocity with the 8208 for the heavier bullets. For 50-60gr bullets, H335 or H322 will probably be the best bet. Only you can determine what you want. For me? Accuracy > all.