I found a better comparison for the SA-18s warhead: the HE round fired by the BMP-1. They're both 73mm, but the BMP-1 has 730g of TNT filler in its HE shells compared to the SA-18s 390g of HMX (which is equivalent to about 660g of TNT).
If the Shilka gets SA-18s, expect them to have a warhead that has similar (probably slightly worse) damage and penetration the he BMP-1s HE rounds