[janoskiss (Steve H)]

Just to put things in perspective a bit:

If a filter blocks 50% of the light that's like going up one f-stop. As far as amount of light per unit area in the focal plane this means: your f/4 would behave more like an f/5.6, an f/5 like an f/7, and more generally a factor of 1.4 increase in focal ratio.

If it has 70% transmission, that's just around half an f-stop: f/4 goes to f/4.7, f/5 to f/6, or a 1.2x increase in focal ratio. If you get a dramatic improvement in CA then it could be well worth it. Also if it's for DSLR photography, then with a good minus-violet the green channel, where often most of your luminance signal is, will suffer very little light loss.

Theoretically, the way to assess whether it's worth using a filter or not is to compare the filtered results with a stopped-down aperture and see which one gives the best image-quality vs photon-quantity compromise.