Overall, not too much. The difference between 15 & 18 is, naturally, negligible. It's why I prefer the Scrapper's over my 27% SDI Invasion Spellbook on my Myst/Weave Mage. The only thing that's really affected would be Word of Death, which already does 780 damage with 160 SDI.
You could put 1/3 on it though. I ended up getting a ring with 18 SDI & I think 20 DI, I had a friend imbue the DI down to 1%, add 17 LRC, and give it 1/3. I haven't used it yet, purely out of laziness.