The biggest reason I say top 6 or bust is because his defense is suspect, and we've already drafted a plethora of very good bottom 6 forwards. Hence the chuckle at the Killorn self assessment, he knows full well that he's underdeveloped in the defensive zone.
Personally I think he resembles Mark Stone. He doesn't have Callahans motor, and despite Callahans propensity to attacking the net Raddysh is almost half a foot taller and heavier and plays more on the perimeter.
I agree on his floor, Raddysh definitely has a role in the NHL. I think Raddysh has two options to making it. Like you said, improve his defense, and then he can eventually be a Callahan type bottom 6 guy. If he improves his defense and his skating, he can become a Pacioretty or JVR type power forward, that is his ceiling. His speed is already above average, just not elite. We haven't had a guy like that since Modin. Even as just a big, complimentary winger, Malone was pretty successful in the role.