@Locurian I'm not 100% sure, but I think it's additive, because if it's multiplicative it would be too underpowered.
For example the RE that increases the SP scaling of thunder clap does so by 8%, but looking at its unbuffed SP scaling in the builder it says it is 1% (basically nonexistent). One such enchant would make it 9% if additive or 1.08% if multiplicative. Since the enchant, when stacked 3 times, actually makes Thunder Clap scale pretty well with SP, I'm assuming it's additive and it ends up at 25%, not 1.24%.
Again, I'm only assuming all "scaling" modifiers work the same way, but we'd need confirmation from the devs or GMs.