There is no point to send your official SAT subject results, as they won’t be considered like official ACT or SAT results. You could mention them in the application if you are listing accomplishments, awards, etc. So, do not take an SAT2 subject test only for UIUC admission. If you needed to take a subject test for another school and received a high score, you can still mention it “unofficially” on your UIUC app, though it probably won’t have much impact.
https://blog.admissions.illinois.edu/?p=24237
https://engineering.illinois.edu/admissions/undergraduate/index.html