I'm not quite sure, but making an account at developer.nvidia.com would probably be a good start, and then either on their forums, or via the "contact" form.
So... How do I report this so Nvidia actually listens?
The small, self-contained reproduction test is great for them to be able to see exactly what you're on about.
I haven't looked at your code, but simply adding an index buffer shouldn't double your CPU-side frame-times I would think...
I ran your test on my PC (Q6600 CPU, GTX460, driver 184.108.40.2068 12/05/2013) and got (from left to right buttons): 77 43 31 83 42