Ok maybe its not relevant, but looking at the frame data, I actually think that prepBB and 6BB are the same speed (i12) but the discrepancy is actually prepAB is 1 frame slower than 6AB (i16 vs i15). Obviously this doesn't matter as much, because either prepBB is faster than normal 6BB by 1 frame, or prepAB is 1 frame slower than 6AB, so you'll just have to adjust the stance entry frames by +1 or not if you want to undersatnd how to use this information.
For more relevant news, after messing around with JG, it appears that 66(B) when jG'd is at least -20, since you can tag 2p raph with a 66K before he can SE evade the high. As long as jG'd 6AB is -5 , (or below) its the earliest prep entry that can actually prep4 AGI raph's 6BB (i12 so the aGI starts at frame 7!). Tested the -5 by inputting jG into grab vs 6A(B)~prepBB, which means the i17 grab trades with the i12 prepBB.
So to sum it up, prep4 aGI is rather slow for an aGI but its tricky to balance because the aGI is considered too "early" for most of prep on block, and yet its too slow if you whiff a prep entry. For example, how many times have you whiffed 3(B), and inputted prep4 hoping that i'll save your butt from an opponent's 3B ? If the aGI frames started earlier it may be the difference.