When I bought Rem's Turkey Express, it came with a 21" bbl, vent rib, double beads & an extra full turkey choke. Nice package & my fave all 'round 12 ga.
Many a turkey's been taken with 2-3/4" shells, but a larger shot payload is more better - the more you can throw out, the better your odds. I wouldn't feel undergunned with 2-3/4" 12 ga #6s & a mod-full choke, but pattern that thing first! & only count head/neck pellets as real hits.
Only way to know, is to pattern a vaiety of loads in your shotgun. I had tremendous variety using the Rem Duplex, Win's 1-7/8 oz loads & Fed's Premiums. Settled on Fed's 2oz of #4s - for my shotgun.
Loads in #6-#4 lead w/a heavy payload seem to work best. Note, no mention of velocity. You shoot a turkey with enough suitably sized pellets & he'll go down.
Velocity loss in a shotgun is around 20fps per inch of barrel lost & really, a moot point.