I think this was said before many pages ago, but for levelling BSM desynthesis past 200, Replica Allagan weapons really have been the best way by far for me, though possibly expensive depending on server. The two materia slots are a big reason why. Make sure you don't accidentily get Replica High Allagan weapons. Those are based on a higher recipe level, and I don't think those would be viable for levelling BSM desynthesis until at least 230, not to mention their cost is completely prohibitive anyway.

Doing at least the weekly Centurio Clan Hunt is good too. 100 seals=Antiquated Hauteclaire for BSM desynthesis.