If you are going to shoot ammunition with plastic wads it is less an issue with longer cones. However, it may still be possible to get an ocassional 'blooper' due to a poor seal in cold weather or with certain less pliant synthetic wad materials.
The older abrupt or steep forcing cones pre-dated plastic wads by almost 100 years and work well with any of the card or fiber wad ammunition. If you are going to use or load card, cork or felt wadded ammunition you will be 100% better off to leave the cones alone.
I can see no reason ever for having a cone over 1.5" in length, ever. Chuck's comments mirror what the old flyer shooters figured out way back when .. a slight improvement in the pattern quality due to less pellet deformation. Remember that for the most part flyer loads at that time used card wadding and did not generally have the shot protected.
FWIW, Kreighoff's have almost no cone to begin with or about that length [1.5"]and many sporting clay type guns now sport similar long cones in their oversized bbls.
Generally speaking, your money would be many times better spent on lessons with a qualified shooting instructor. That is actually the best money one can spend on shooting, if we are to be perfectly honest about it.
