That is valid and I don't for one moment dispute that Tua is far superior as a QB to Winston on an intellectual level.
The reality is that many passing leaders in the modern game have been consistently good QBs for a number of seasons (Brees, Brady, etc). On the other hand, there have been counter examples aside from Winston (Schaub, Gannon, etc.).
As far "efficiency" is concerned, it isn't a bad thing as long as there is context. A Toyota Corolla can be very efficient but not my first choice for a Nascar race.
I actually believe Tua is much more than simply "efficient". I don't really even like the term as it relates to QBs. It is just a compilation of statistics, subjectively weighted.
It has the connotation of a non aggressive, check down, play it save trigger man, when that is so not the case with Tua.