it would be completely fucked up for you to get a piece finished by someone that didn't start it. regardless of them "reaching a certain level".
if you're going to a quality tattooer and you sign on to get work from them, you have to see it through unless you're unhappy with the work for some reason. going to someone else to get a piece finished just because you don't want to wait is about as disrespectful as it gets. don't get tattooed if you're going to pull some bullshit like this. people have no respect for tattooing anymore. this isn't a situation where you buy your xbox at target and buy the games at wal mart. this type of attitude pisses me off. why get tattooed by a given artist to begin with if you're going to have someone else finish it? just go to that motherfucker in the first place.
whoever you choose to go with be sure they're who you want. book a couple appointments at once if it's large scale work and make the commitment the same way your artist makes the commitment to apply the work to your body.
I've had this happen to me more than once not because a client couldn't wait but because they didn't want to pay my very reasonable hourly rate to get the work finished. they'd get their boy down the street to "color" a piece I drew, lined and started. this type of shit will earn you a punch in the mouth if I see you in public.