Don't mention Tentacruel when talking about Psychic, as both Psychic moves do exactly the same damage to it (46.15 - 54.94%).
Hemp Man you are right Psyshock hits Gyarados slightly harder so mention this too. Gyarados stays for sure, as it still common enough, and very deadly if played right. I insist on mentioning Cloyster and Conkedlurr, as Slowbro is one of the best counters around for those, even if they are somewhat rare.
Finally, i believe that papai noel is right, and 19 Speed IVs should be the standard on defensive Slowbro, as it loses nothing by doing so, while faring better against Payback Conk, however rare that may be...
Part of the OU QC team, message
me for a check!