ARB_texture_rg+FP16 on NV40 based cards

to my surprise a GF7900GT exposes ARB_texture_rg with Forceware 275.33.
I tried to create a GL_R16F texture and attached it to an FBO. The FBO is considered ‘complete’, no errors are thrown.

Unfortunately, the rendering gets broken with this format. As soon as I switch from GL_R16F to GL_RGBA16F, everything works fine.
The shader only writes to gl_FragColor.r, ColorMask is set to (1,0,0,). The last step is to call glReadPixels() on GL_RED with GL_FLOAT type.

Is this a bug, or is this some explainable behaviour?

Machine: WinXP64, Forceware275.33, GF7900GT/GTO

Additional Info:

Rendering (or is it the sampling?) into GL_RG16F is horribly slow in that GF7900, although the GeForce6 programming guide says that it is a nativly supported Format that can be rendered and filtered.

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