Kedako and Ruu have been doing a lot of testing and research concerning the system esoterics of Guilty Gear Xrd -SIGN-, and I wanted to make this post to translate their findings and put them somewhere for reference.
UPDATE: Ruu’s blog now has complete frame data for every character, including Sin, Elphelt, and Leo! I will update this post periodically as more translations become available.
All characters once again have 420 health, down from 460 in AC+R. Hellfire state kicks in when 337 points of damage have been dealt: basically, when you have 20% or less health remaining. Kedako tested this by playing Chipp in sparring mode and throwing shurikens(which do exactly 1 damage) 420 times.  Now that’s dedication
Defense modifiers
Sol 1.00
Ky 1.03
May 1.06
Millia 1.22(up from 1.21)
Zato 1.09(up from 1.06)
Potemkin 0.94(up from 0.87)
Faust 1.00
Chipp 1.30
Axl 1.06
Venom 1.06
Slayer 0.96
I-No 1.06
Bedman 0.94
Ramlethal 1.06
Sin 1.05
Elphelt 1.00
Leo 1.00
As you can see, defense modifiers are mostly the same, except for the aforementioned 3 characters, who had theirs nerfed. Between the 3, Potemkin’s defense nerf is the most significant.
Wakeup times

Character Faceup Facedown
Sol 53f 49f
Ky 51f 49f
May 53f 50f
Millia 53f 51f
Zato 53f 50f
Potemkin 52f 50f
Chipp 58f 52f
Faust 53f 57f
Axl 53f 49f
Venom 49f 54f
Slayer 54f 48f
I-No 52f 48f
Bedman 52f 58f
Ramlethal 53f 51f
Sin 58f 49f
Elphelt 55f 55f
Leo 56f 54f
Roman cancel startup times and other info. There is a ton of info here, but I’ve only been able to translate the following.
YRC: 6f startup followed by 18f of screen darkening. Also has 2f of invincibility
RC: 1f startup
PRC: 15f startup
Ruu tested the red roman cancel mechanics, and came to the following conclusions. Red RC has 1f startup, and slows down the opponent’s speed by half. In addition, the red RC adds extra frames of hitstun or blockstun. The extra stun increases depending on the level of the move that connected beforehand:
Lv1 18f
Lv2 22f
Lv3 26f
Lv4 32f
Lv5 36f
This is why certain combos only work when you add in a red RC. Typical example is Sol 5s5h Bandit Bringer, which does not work normally. But 5s5h RC Bandit Bringer does work.