Would you see different results with a full spectrum conversion using an external filter compared to a dedicated conversion to a specific wavelength with all else being equal? For example, would a full spectrum converted camera with an external 590nm lens filter and the same camera converted to 590nm via an internal filter yield similar results? Assume that both scenarios used the same lens and settings.
If the internal high-pass infrared filter used in a camera conversion were the same manufacturer and wavelength as the external high-pass infrared filter used on a full spectrum camera, you would see no difference in the resulting image.
Filters from different manufacturers can produce different results, since they may have slightly different transmission graphs, but they would be very similar if they were both high quality filters from known filter manufacturers. Low quality filters are another story; as they have shallower cut-offs and are prone to pass visible light.