Yes, with most other high CRI / low R9 emitters I’ve tested the filter with I usually get R9 coming in around the 70s. The Zircon filters do seem to be particularly suited to the LH351D’s phosphor mix.
I don’t notice any diffusion of the beam. It’s slightly less intense due to slight loss of lumens (in this case about 15% flux) but the hotspot is the same size.
The output is being slightly lowered but the test isn’t faulty. A spectrometer compares a number of color samples to a set of known samples under a 100 CRI (incandescent) source and figures the difference. More deviation results in a lower R (x) number. If the spectrometer is reading the R (x) value as high, in this case it means the filter is causing the color to be rendered very closely to how it appeared under a 100 CRI source (with all other considerations such as CCT and tint taken into account).
If your filter caused the light to become washed in a pink hue, you probably used a much too aggressive filter and went too negative on the Duv. I personally find an LED that is too pink to be just as offensive as a green one.
Slicing the dome of the LH351D does improve the tint slightly but not to the same degree as a filter. It will also result in a much more aggressive shift in color temperature. You will also lose some luminous flux, just as with a filter (though slicing will result in an increase in intensity).
Also keep in mind that while slicing the LH351D results in a decrease in Duv, this won’t be the case with all LEDs. With most emitters, Duv will increase significantly after sliced or dedomed.