I'm not sure on the proration, but I tested its full damage on Tager in the corner a while back and it seemed to deal around 6k with the full 12 nails in stock. When I throw it out after the round is over and I dont have any nails left, its about 2.7k. Best of all is to corner him, Steel Rain, then Daifunka. I think because it black beats from too many hits, Tager will be able to instant tech and block, but the last hit of Daifunka will be unblockable and land for a new 1 hit at an unprorated 3.3k I believe, in addition to the ~5-6k from the first 60 hits or so. Against Rachel, she is grabbed for the last hit but flies out of your hands and the last hit whiffs. Messing around with it in training is fun, I recommend experimenting.
I tried RC'ing the command grabs, and I do not believe that between startup and the nails following your opponent down that it will combo even with perfect RC and super timing, but dont hold me to that.
New idea: in frkz, web nail, dash cancel to the ground and super?
Final note, if you want to see an interesting steel rain glitch/bug/odd-game-mechanic, then do bang vs bang and record a steel rain. Have the cpu do playback and after the cpu starts the super, neutral jump and guard point asap. Once you get the first hit to activate the j.D guard point, just wait. Enjoy and lol.