The best you can do in terms of reproducing luminance is accumulating the effect of the strength of the ray on surrounding scanlines and also take earlier/later pixels in the scanline into account. Which basically amounts to a large, luminance-dependent, anisotropic Gaussian masked by the shadow mask.
Once that is taken care of (and the Lottes shader does it well IMHO), the other thing you need is a good, fast, high-resolution and high-contrast display.
Anyway, the reason the 4:1 mapping was looking much better than 1:1 is because the texture lookup still wasn't quite right. I now implemented an ugly hack in the shader to make it work correctly (no idea why this is necessary), and after some more tweaking the 1:1 output also looks neat:
Now after spending hours investigating and tuning this CRT shading thing I want to play a SNES game.