Great on offense 65+ points, average 40-45 points for a 1D. Great on D is harder to quantify and more subjective, Foxx/Hedman are examples. Chych/Yandle/Sergi fall into above average point production but not elite. All 3 are average on D, not great. Sergi is 26 so his D game can improve, as somebody mentioned, D peak later then forwards, closer to 30.
TB kept an older MCD and traded Sergi, part of it was cap reasons. If Sergi was truly a 1D in the making, I doubt they would have traded him, they would have let MCD go instead. 26 year old 1Ds on long term deals don't get traded unless it is a monster return. I think TB got a good return but not a 1D return, because he isn't a 1D (yet). TB had a good look at his game for years to make this determination, that team won cups, the GM isn't stupid.
Last comment, I am pro Sergi, he has been our best player this year, I think Marino and Durzi coming back will take some pressure off of him, allow him to free wheel some more and his scoring could pick up. The 7 game home stand is very important for the season, lets go Yotes!