I'm not an expert horner, however I have made a few. IMHO, you would be better off using a small horn, as opposed to cutting down a larger horn. The medium to large horns sometimes have a very long, solid tip, and thicker walls, which can be a problem when attempting to make priming horns, though the tip can be cut off, above the the end of the cavity. IMHO, using a larger horn to cut down would be more work to thin the tip and walls of the horn. That said, if that is all you have access to is a medium/larger horn, it will work.
That said, IMHO, priming horns are more of a PITA to use than priming from the main horn. I doubt that there will be any appreciable difference in time from spark to discharge, even when priming with 2F powder.