To be honest I really don't think it's worth sprit bonding anything level 50+.

Being as converting doesn't give grade 5 materia at all your better off sticking to 48 gear which is super quick to do. Also has chances of grade 4 materia. Took me about a day to spirit bond 88 peices. More than enough materia to min meld crafting gear at 60. (I'm grand master of hand so I would say this is definitely more effective.)