I can say from I experience that most performance issues related to grass come from overlapping transparency rather than from the amount of tris or even other types of shader complexity. Basically, you should try your best to leave as little empty space in your alpha as possible, try to not have many many planes overlap each other (if possible). Your LODs should be about reducing this issues rather than reducing polygon count (as an example, my LOD0 has bended, intersected grass planes to look good from above. My second LOD removes the intersecting plane because it will no longer be looked from above. This means less overlapping transparency).
If your material is taking a performance hit (wind, SSS, etc.), It’s not a bad idea to use a different material for your LODs, a cheaper version which won’t look very different because it’s not being looked at closely.
I also set my LODs to not cast any shadows and only my base mesh casts one so only the closeup has the detail it needs.