If your shockwave is a planar projection of a spherical shockwave, you should use a planar texture for it.
In other words, if you have rendered the shockwave animation from an actual 3D explosion, you should use the 2D shockwaves to get it in FS2_Open. Those are always seem from normal direction, so they will look best in the game, much better than disk-shaped 3D shockwaves.
Now, if someone actually makes a spherical in-game expanding shockwave model with distortion and maybe some sort of fake fresnel effect/glow, then it would probably look better than the 2D shockwaves.