Yep, FSI is considered a standard racial so you have it permanently on the character, but slam has the visualrace flag, so you gain or lose it with an illusion.
Keep in mind that Regen is also like this, but troll race comes with a visual race regen modifier of 1HP that doesn't stack with being an actual troll
[You must be logged in to view images. Log in or Register.] So a guised troll actually still has their full racial regen, an an enchanter in troll form gets a bonus 1HP regen.