You must have never heard of the Mog wardrobe, which basically gives you 160 usable slots to put gear in.
Apart from that, as others have said as well, you don't use Siphon until you at least have used the MP it fills.
If you don't, you are wasting it.
Using slots for extra MP is completely useless, especially
since that MP disappears again when you unequip the item.
Oh, and FYI, my summoner currently uses 103 inventory slots devided over the 2 storage options.
More if meds and food come into play and I am still a long way from 100% efficiency.
As has been said to you in another thread: Stop forcing your beliefs on others, the TS asked for optimal options, not for poor man's choices and not for advice from someone that doesn't want to play a job at its optimum efficiency.
Once again, in a nutshell, the only things that matter for Elemental Siphon in order of importance:
- Stuff that enhances it (Which includes, as many of us have learned from this, some items that have been known to enhance weather and day bonuses.)
- Summoning Magic skill