If the OP is going to shoot the gun very much and does not reload, that's good reason to use 2 3/4" factory ammo. RST's are great, but they're at least twice as expensive as the 20ga target loads you can buy at the big box stores. Most likely 3x by the time you figure shipping.

I'm not normally a fan of lengthening chambers, but in the case of a Nitro, as long as you're not going to shoot short magnums or other very hot loads, it would probably make sense for someone with a 20ga who does not reload.