OP, you should have responded by asking the BLM to Manafont so that you could cast some spells free of MP.

Anyways, there's more than enough gear available now to make your avatar completely free without using storm spells. Also, they don't really boost Siphon as much as you think because you have to subtract the storm spell's cost from the bonus MP gained. SCH's main benefit to SMN was Reraise, but now that Cait Sith provides it RDM is by far the better subjob.