I've long since driven myself insane soft resetting for a 6 IV Kyurem in ORAS to turn into a mix attacking Kyurem-Black Just got one with 31/31/31/26/31/31 with a mild Nature. Even though I wish it had that 31 Special Attack and want to keep looking for the perfect 6IV, for my own sanity I think I should at least ask if the 26 will keep my IB and EP from doing what it will need to do for OU
I can't remember if OU is set to 100 or 50, I will talk as if it is 50, but it won't matter much. With no investment the 31 IV stat in special attack is only 2 higher than the 26 (140 vs 138) so it won't really matter and the mild nature means that at lvl 50 with the mild nature it is 154 vs 151 but even that isn't much. There is a difference: mild nature 26 vs 31
0+ SpA Teravolt Kyurem-B Ice Beam vs. 4 HP / 0 SpD Thundurus: 152-182 (98 - 117.4%) -- 93.8% chance to OHKO
0+ SpA Teravolt Kyurem-B Ice Beam vs. 4 HP / 0 SpD Thundurus: 156-186 (100.6 - 120%) -- guaranteed OHKO
but for all the effort you can probably keep the 5 IV