Talking about Ryo's SDM in KOF 2k2. You say C~A Hard C, then quickly A. That is not the question. Actualy the speed is the same as K' MAX2 (qcb + C~A) and I'm sure you already get it.
The question aren't the buttons but THE JOYSTICK. The command is qcf + C~A but Ryo has another move which is performed by qcf + C (ko 'ou ken C version, short ranged projectile) so you need to enter the command while you're recovering from another action.
The command when you use it "standalone" is:
a _ perfectly hidden_ qcf while you're recovering from another action and then C~A (when you get the mobility). When I mean a perfectly hidden quarter circle forward I refer you must even release the joystick to neutral stance and so you can finally press C~A. For example: do the MAX activation, when you watch that pose, perform the qcf and don`t forget to release the joystick, then when that pose ends C~A .
You can even perform it after a jump/hop. Jump and near of the end of the jump/hop qcf and release the joystick and exact you land C~A. The SDM can be performed after a roll, backstep, during the last frames of an attack that _doesn't hit_ (let's say the final frames of Ryo's crounching C) and after you block an attack (yes, after you block an attack) as well.
Unfortunatly the hidden qcf must to be so perfect (the joystick should return at neutral and stay still). If you do the qcf, releasing the joystick and it gets shaking a little bit, the SDM won't launch at all. I usually shove the joystick with the palm of my hand (instead of using my fingers) in order to appropriate works.
Performing the SDM in a supercancel (after his dragon punch) is another story. In fact when you're in MAX mode you are not able to do the ko' ou ken in a "free cancel" (cancelling a special move by other one) so dragon punch cancelled by ko' ou ken is not allowed. For this reason release the joystick is not a requirement. The trouble comes when you do the move in a fast way. You possibly get buffering a Haou Shoukou Ken instead. To prevent this I suggest carefully perform the dp, release the joystick at neutral trying to don't get a shaking effect., let Ryo hit the first hit of his dp, let Ryo travels a little bit in the air (yes, a little bit in the air) and then qcf + C~A. I've only gotten this 2 times in my whole life. It's the hardest way to pull it off if you ask me.
Ryo has invincibility during the startup flash (except for throws) but during the forward dash you're FULLY INVINCIBLE and yes is _unblockable_. The move can be reversed and the autoguards can stop it, though.
I hope this can help you.
P. S. English is not my native language, so I spent a long time typing this LOL
Regards from Mexico.