Hi Welcome
You are NOT casting Shadow Illusion, you are casting Silent Image. When employing Shadow Illusion, Silent Image explicitly allows SR (even if the spell being mimiced does not). Since Silent Image NOW allows SR to negate, Spell Immunity cast to protect against Silent Image will completely negate any and all spells mimiced using Silent Image. Using Shadow Illusion to alter a Silent Image spell does not change the fact, AT ALL, that Silent Image is a spell.
[spoiler]
Really? Going through the mental exercise of working through the interactions of various rules pieces in the CO board now gets so easily blasted with an asinine Hi Welcome?
Anyways, I'm not wedded to the idea of Spell Immunity not affecting SCM's shadow illusions. Just trying to work through the pieces and was using the forum as a sounding board and aid.[/spoiler]
1) I agree. The SCM is NOT casting Shadow Illusion.
2) Shadow Illusion(Su) just modifies the Image spell allowing it to mimic other spells.
3) Since the modified Image spell functions like the various Shadow spells (which grant SR) the modified Image spell will as well.
4) Thus, Spell Immunity (silent image) would be viable against the SCM's shadow illusions. (Since the ability only modifies the spell and does not convert it to a Su ability).
Pretty much what I figured from the beginning and how I'd run it as well. Just wanted to challenge that view of the rules to see what holes could be poked in it.
X-Codes, in spite of the "Hi Welcome" thanks for your feedback.
Cheers.