Assuming that 2.5 is given as the angle per side of the cone it is not a particularly long cone, being only about 3/4" long. If it is the included angle then the cones will be around 1" long. About 40 yrs ago I bought a little Italian built Richland 707 3" 20 gauge which had 1" cones. The bore was constant from cone to choke though.
I have been given dimensions on a couple of Lefever's on which the barrels had step downs in the bore connected by short cones. I have never had a Lefever so bored nor do I know of anyone who has ever found one with a constant taper from Breech to Muzzle as their advertising implied. All I have had any knowledge of, including those with the step-down bores, still had their main choke at the muzzle, in the form of the "Taper Choke".
I will personally agree with Rocketman & say any benefit derived from this boring will be purely & entirely Psychological.
