I can't say that I've observed tighter groupings at 50m with either of my guns. Both using the same ammo... bastard .28s and bioshot .3s and .32s
But to do a scientific test, I would need a gun vise and a 50m indoor range to eliminate wind and human factors. I personally would love to do this test or see someone do this test to provide definitive proof that a longer barrel or a tightbore barrel makes a difference over a standard bore and length barrel.
Unfortunately, the ammo is also not consistent enough to do such a test, of course the law of averages can narrow it down for you but the more consistent the ammo, the lesser margin of error you're going to have.
I futz with V2s, V3s and V6s. I could be wrong... but probably, most likely not, as far as I know.