In my experience using the tform commands to rotate the individual quads is almost twice as fast as using sin/cos etc... You may get some speed up using Lookups for sin/cos but very little on modern hardware.
You are correct, the slowdown over 2000 leaves (4,000 polys, 8,000 verts) is only about 4-5 ms on my machine using Tform (which I can live with). Whereas doing a sin,cos operation (just to rotate the verts around a center point) slows the operation down 30-45 ms.
Tform is the way I am going.
I found look up tables were slightly slower. I agree with the tform commands being faster too.
My findings are the same, lookup tables are a bit slower because you still have to do a lot of calculating after you get the sin/cos lookup to rotate the verts and such. Tform converts it into the mesh's local coord, and my merely move the vert to where it should be (so 1 tform, and an XYZ addition to each vertex).