[Jasper_]

You shouldn't use strips, they're slower than triangle lists on most GPUs.

If by alpha cut you mean "discard", that's going to be much slower than two triangles. Two triangles will have a tiny bit of quad overshading on the seam, compared to a full extra triangle's worth in the alpha cut case.

[quadcore]

Yeah discard use to be slow because it flushes pipelines or mess with branching predictions I don't remember which, I just assumed they "fixed" that by now.

[Jasper_]

No, it's not either of those, it's just launching useless threads, plus all the down-stream effects of launching useless threads, e.g. if you have blending on, that will block the ROP unit which needs to wait for the threads for a given pixel in-order. If you have depth write on, that will move the write to late-Z.

More vertices is not a big problem, doubling your vertex count is not a big deal, since most GPUs process vertices in groups of 32 or more, and whether multiple instances get packed in the same group depends on the GPU vendor.