I believe his star moment was winning golden boot last season. And if you believe @Leif then he is one of their all-time greatest scorers. Does that say more about Roma or Dzeko? I can't say, don't watch Serie A, but consider his competition were Higuain, Mertens, Belotti, Icardi. Not bad.
Now one more thing to consider. The names you mentioned were bought as big money starters, star men of their teams who will be playing every game. We already have our main men in every position, the aim of signing players like Dzeko is to strengthen quality in depth which is as, if not more important for us.