I really want to use dual katanas on my stealth character, and Backstab will let me, but I notice the perk Assassin's Blade will let me do x15 damage, which might come in handy sometimes, although it only works for daggers.

Assuming I was to use a dagger+katana, would the perk still apply if I stealth-kill someone?

Also, if I use the double power attack while using certain weapons, do certain weapons make the attack slower/faster?

I'm honestly not sure when you would need a x15 when you can already use a x6...

Assassins Blade only works with Daggers...
So it may work if you have a Katana in your off had, as long as the blow was dealt with only the Dagger....

ohh, and if you wear a set of gloves from the Dark Brotherhood Uniform, that x15 should double to a x30...
With a decent Dagger you can one shot most things...

Except dragons, they can sense if you are near them so sneak attacks are very very very difficult on them

you can pull it off with the shadow warrior perk.