I know a lot here hate on me for saying this but here a goes.
OP, the 18.5 inch for skeet is fine, as long as you do your part. I notice little difference between my 500 on the 18 vs the 28. Also, testing the reach theory out with birdshot vs a 28 inch barrel with a modified choke, we tested on self pull, and saw how far I could connect with clays...... It turned out to be about just as far as the other shooter that had a modified choke in his gun.
Does the 28 inch barrel give you an advantage in shooting skeet? Perhaps, but I'd like to up my proficiency with my HD gun on tiny 4 inch clay discs flying through the air, so if god forbid I ever have to use it in an HD situation, that big torso going a fraction of the speed of the disc will seem a little easier to hit (in theory).
To each their own, you'd be surprised at what scores you can make with an 18 inch, and you may be amused by the people you'll upset when you do outshoot their 1-2k gamer shotguns in a friendly match.